The basic information I would ask for starts with the type and thickness of all the layers and components in the mattress listed in order from the top down or the bottom up so you can make sure they add up to the total thickness of the mattress and nothing is “missing” (see this article which you can print and take to the store).
Once you know the type and thickness of all the layers and components then you can identify whether the mattress has any obvious weak links that can affect durability and the useful life of the mattress. The most important part of this is knowing the specifics of the upper layers (the top 5" or 6" or so) because these are usually the weak link of a mattress (a mattress will tend to soften, break down, or compress from the top down). Heavier weights will tend to need thicker layers of higher quality materials in the upper layers of the mattress because they will sink into a mattress more deeply and put more wear and tear on the deeper layers than lighter weights.
If there are polyfoam layers (regardless of type or the proprietary name or whether it’s plant based or not) or memory foam (again regardless of the type or the proprietary name) I would want to know the density and that it’s made in North America or that it’s CertiPur certified if it’s not. If the foams in the mattress are sourced in Asia or China then I would also read post #6 here because materials or components that have been compressed over the longer term in shipping or storage may have a higher chance of durability issues over time.
As a general guideline I would want to make sure that there is no more than “about an inch or so” of lower quality or “unknown” polyfoam or memory foam in the upper layers (top 5" or 6" of the mattress). Once you are at about 2" or more of lower quality or unknown polyfoam or memory foam then the odds are higher that it will become a “weak link” in the mattress and affect durability and reduce the comfort and support life of the mattress (which isn’t covered by a warranty).
For polyfoam comfort layers I would use a guideline of 1.8 lb density or higher in a one sided mattress (higher weights would do better with higher density in the 2 lb range or higher) or 1.5 lb density in a two sided mattress. In the lowest budget ranges where there are no higher quality materials available then I would consider 1.5 lb polyfoam as long as the mattress is “price appropriate” and you are comfortable with the tradeoff between durability and price.
For support layers I would use a guideline of 1.8 lb density for polyfoam unless once again you are in a lower budget range in which case I would use 1.5 lb density as a minimum density polyfoam support layer (support layers are not usually the weak link of a mattress). Using slightly lower quality/density support layers is a better budget tradeoff than using lower quality materials in the comfort layers. Once again though if the comfort layers are thinner and the support layer is part of the top 5" or 6" of a mattress and/or you are in a higher weight range (higher than @ the low 200’s) I would consider using higher quality/density support layers as well. I would also want to make sure that any polyfoam is manufactured in North America or if it’s not that its CertiPur certified (or has a similar or better certification such as OekoTex).
For memory foam I would use a minimum guideline of about 4 lb density or better and for higher weights I would lean towards 5 lb memory foam or higher and reduce or minimize lower density memory foams. If you are in the very lowest budget ranges and there are no alternatives to using memory foam that is lower than than 4 lb. density in layers more than about an inch or so strictly for reasons of budget (there would be no other reason to consider this) then I would consider an alternative material other than memory foam that will be more durable or if you do choose to use memory foam in the 3 lb range (and no lower) I would make sure that it’s “price appropriate” and that you are OK with the tradeoff between lower durability and price so that your expectations of the useful life of the mattress are realistic. Once again I would also want to make sure that any memory foam is manufactured in North America or if its not that it’s also CertiPur certified (or has a similar or better certification such as OekoTex). More than an inch of memory foam that is less than 4 lb density would be much more risky in terms of the premature loss or comfort and/or support.
With latex I would want to know the type and blend of the latex although all latex is a good quality and durable material relative to other types of foam. There is more about the different types and blends of latex in post #6 here.
I would also want to know the type of fabric and the quilting material (if any) used in the mattress (generally natural or synthetic fibers or foam).
Outside of testing an innerspring mattress to make sure that it’s a good match in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) more detailed information about the innerspring isn’t usually that important because they aren’t normally the weak link of a mattress and you can “feel” the properties of an innerspring as part of the overall mattress construction when you test the mattress. Some of the innerspring specs that can make a difference and help you make better comparisons between mattresses are the type of spring (Bonnell, Continuous Coil, Offset, or Pocket Coil), the height (so you can add the thickness of all the layers and make sure there aren’t any layers in the mattress that are missing in the specs), the gauge of the wire (lower gauges are thicker and stronger), details about any zoning, the type of edge support, and the number of coils (which by itself is not really meaningful or a useful comparison between different types of innersprings), and the number of wire turns in each coil (which usually won’t be a spec that is available).
In most cases though you won’t be able to find out all this information and this can be much more complex than necessary because it’s the layers above the innerspring that are much more important in terms of durability and the useful life of the mattress but some basic innerspring information (type of coil, coil gauge, and the number of coils) about the springs along with the height to make sure you have information about all the layers in the mattress can help you make more apples to apples comparisons between mattresses in terms of cost and value. There is more information about innersprings in this article and in post #10 here.
If an innerspring has a foam surround for edge support then I would also want to know the density of the polyfoam it uses because lower density here can also soften and compress too quickly … especially if you sit on the edge of your mattress. Once again a density of 1.8 lbs or higher would be a good guideline. There is more about innerspring foam surrounds and edge support in post #2 here.
It’s usually much simpler to deal with experts that have your best interests in mind and already know what you would otherwise need to learn in what can be a very complex area of study than to go through a longer learning curve and “become the expert” yourself because in some cases too little information and too much technical information can both lead to some poor choices.
Of all of these “specs” … the specs that affect the quality/durability of all the foam layers in the upper layers are the most important. In many cases and with many manufacturers they are also the most difficult to find out. Most people that sell mattresses have very little knowledge about even basic foam quality specs. This is usually enough to make meaningful assessments and comparisons or identify any potential weak links in a mattress. For those who want more detailed information about the durability and useful life of a mattress relative to each person then post #4 here and the posts it links to has much more information about all the variables that can affect durability.
For those who are more concerned about safety and VOC’s then in addition to certifications for the foam materials (generally CertiPur for polyfoam and memory foam and Oeko-Tex or Eco-Institut for latex) I would also ask about the type of fire retardant materials used (see this article and post #2 here) and about the glues that are used (solvent based glues generally have more VOC’s than latex based glues).
It would be rare enough for a mainstream retailer to even know this much about their mattresses and to try and find out more detailed specs (such as the percentage of plant based polyol replacement for any plant based foam or the percentage of gel in a foam) would generally be too complex and restrictive and wouldn’t have much meaning or potential benefit for most people anyway.
In most cases … the most important information you need is the thickness, type, and quality/density of any foam layers in the mattress because this is normally where you will find the “weak link” of the mattress.
While there is no way to know for certain how long any mattress will last or maintain it’s comfort and/or support for any particular person or how long it will take before they cross the thresholds between sleeping well on a mattress to sleeping “OK” to tolerating a mattress to finally deciding to replace it because there are too many variables involved that are unique to each person … if a mattress is well inside the comfort/support range that is suitable for a particular person and isn’t close to the edge of being too soft when it is new and meets the minimum quality/durability specs that are suggested in these guidelines then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer.
It’s always more realistic to think of about 10 years as a reasonable expectation for any mattress no matter what the quality or durability of the materials and then treat any additional time after that as “bonus time” because after about 10 years the limiting factor in the useful life of a mattress will often be the changing needs and preferences of the person sleeping on the mattress and even if a mattress is still in relatively good condition after a decade … a mattress that was suitable for someone 10 years earlier may not be the best “match” any longer.
Having said that … if a mattress only uses the highest quality and most durable materials and for people whose needs and preferences or physical condition or body type hasn’t changed much over 10 years then “bonus time” or even “extended bonus time” with more durable materials such as latex, higher density memory foam or polyfoam, natural fibers, or other high quality and more durable materials that soften, compress, or break down much more slowly is much more likely and you will find some people who have slept well on some of the most durable mattresses and materials for several decades but these are the more the exception than the rule.