Hi pinkpeonies,
In very general terms you would need to make sure that you find out the type and quality of all the materials inside your mattress so you can make sure there are no lower quality and less durable materials or weak links in any mattress you are considering (see this article).
In more practical terms this would mean avoiding any of the major brands or any other mattress where you aren’t able to find out the type and quality of the materials inside it (see the guidelines here) and I would focus more on retailers and smaller manufacturers that are more knowledgeable and transparent and will provide you with the information you need to make an informed choice.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses regardless of the name of the manufacturer on the label.
The mattress shopping tutorial here also includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly help you learn how and why to avoid the worst ones.
Phoenix