I think the easiest thing to do is just look and see if it has a stamp on it. Very often it’ll just tell you. Look for “full grain leather” and be wary of “made with leather”. Another thing you can do is to really acquaint yourself with what a full grain leather belt looks and feels like and then you’ll have a feel for the weight, stiffness and finishing. You can also look for ones that look a bit worn, with slightly deformed holes, a little warping, scuffed edges or tooling etc. If it looks worn, it’s already withstood the test of time, and real leather can be re-dyed and reshaped. Obviously don’t buy something that looks like it’s falling apart though, cracking and rips aren’t worth the hassle if it’s not something sentimental. You won’t necessarily find a good leather belt every time you go to a thrift store, but it’s definitely something to keep an eye out for and you never know.

You can also always buy a bigger belt, because full grain leather can be cut and the edge finished with hobbyist crafting tools.