@Totalanimefan:
Usually by taking your time and making sure spinal alignment is supported. The mattresses in the store are usually pretty broken into (I walk on my store mattresses to make extra sure of that lmao fluffs the materials when they're new), so you can at least get a first impression of what kind of comfort level the mattress has, and if you have a GOOD sales person who can determine your alignment (without stupid surveys and machines that hold no true merit) then you can get a good idea if its going to continue to be comfortable after your muscles settle into it!
If its a brand new mattress, the materials have to be broken into like new shoes. They're often more stiff, so thats why we say give it 30 days. muscles adjust and so does the mattress. But always always go with a store that has the best "trial period" and no restocking fees, cause there's always a chance for exchanges to be needed.