Hi dzignr_tastz,
It could be argued that you can’t (even with large manufacturers) and the same would hold true for any consumer purchase where you would need to tear it apart to really know what was inside it. You could also weigh every mattress that you purchased to see if the specs came out close to the weight but in practical terms you would be reliant on the reputation and integrity of the manufacturer. It would be a big risk for a reliable manufacturer to misrepresent foam densities because they can be calculated from the weight and volume of the material and it’s much easier to lose a good reputation than to develop it.
This is one of the reasons that I so often suggest that who you buy from and their knowledge, experience, transparency, and integrity can be one of the most important parts of a successful mattress purchase.
You can’t “feel” or see the quality of a material and the “suitability” or “comfort” of a mattress in terms of PPP is very different from assessing the quality/durability of a mattress because even very low quality materials can feel the same as higher quality materials in a showroom or when the mattress is new … they just don’t last as long. Suitability, durability, and the other parts of each person’s personal value equation are all important parts of the “value” of a mattress purchase (see post #13 here).
I read it when you first mentioned it … and like others that are probably reading it as well I’m still waiting for the “punch line” 
No problem at all … you are asking some good questions and I don’t stray far from the forum anyway … on weekends or weekdays.
Phoenix