Hi Dave88lx,
I completely understand 
It’s fairly simple to test a mattress for “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) but the most difficult part is often making sure that the mattress uses good quality and durable materials so that the feel and performance that you originally purchased has higher odds of staying that way for longer periods of time. The challenge with this is that you can’t “feel” the quality/durability of the materials so you are dependent on the retailer or your own research to find them out. Lower quality materials in the comfort layers especially will soften and degrade more quickly and the more rapid loss of comfort and support that goes with this isn’t covered by a warranty. Finding out the quality of the materials in a mattress is the part of mattress shopping that can be the most frustrating because most retailers know very little about the difference between higher and lower quality materials and it’s more difficult to find this information or find retailers that are familiar with it in the mainstream industry … even though the useful life of a mattress is one of the most important parts of an informed purchase.
With memory foam I would look for a minimum of 4 lb density with the exception of a thin layer that was less than an inch or so. You can see the memory foam guidelines here.
With polyfoam I would look for a minimum density of 1.8 lbs except in lower budget ranges or in a two sided mattress where 1.5 lbs would be OK. Once again a thin layer that is around an inch or so of lower density polyfoam would also be OK (this is most commonly seen in the quilting in the cover.
With latex … any type would be a good quality material in terms of durability.
Density is the single biggest factor in the useful life of a foam and keeping to these guidelines will give you much higher odds that your mattress will have a longer useful lifetime. Higher quality/density materials are usually more costly so this can also be part of making meaningful “value” comparisons because higher quality materials are (or should be) more costly than lower quality materials.
The mainstream mattress industry is just not very transparent or even knowledgeable and finding good information about the quality and durability of the materials in a mattress … especially the foam used in the comfort layers … is one of the biggest challenges of an “educated consumer”.
It’s often a good idea to call a retailer or manufacturer before you go there to make sure they are transparent (and preferably knowledgeable as well) about the materials in their mattresses and even better yet if you tell them your minimum criteria for foam densities (4 lb memory foam, 1.5 - 1.8 lb polyfoam or latex) you will quickly find out how much they know about the quality of the materials in their mattresses.
Most of them will go “huh???” or ask you if you are an engineer as if engineers are the only ones that want a good value and durable mattress. Another common response will tell you that you need to just come in and test a mattress and that foam specs aren’t important … but the truth is you need to do both if you want the most suitable mattress (that you can test) and also good quality and value (that you can’t). Another common response will be to talk about the warranty as if that is a reliable way to know about the quality of a mattress but warranties only cover defects and don’t cover the softening or breakdown of lower quality materials and the loss of comfort and support which is the main reason a mattress needs to be replaced … not defects. Warranties have very little to do with the useful life of a mattress in other words and are mostly about marketing and justifying higher prices.
An equivalent to not knowing foam specs would be buying a computer where the salesperson couldn’t tell you about the type of CPU, the size or speed of the hard drive, or the amount of memory in your computer. They may also try to sell you a computer that uses a lower performance CPU or less memory or a smaller hard drive at a higher price than it is worth if you don’t know that these are important parts of the “value” of a computer. These are “basic” specs that will make a significant difference in the quality and value of a computer … or a mattress.
If you follow the steps in the guideline post one by one you will have much higher odds of ending up with a great purchase although I fully understand that the current state of the industry doesn’t make this nearly as simple and easy as it should be.
Phoenix