How do you tell if a stud belt is fake or not?

Fake as in materials or brand ?

if you mean materials, a fake or cheap version is usually Pu/Plastic instead of Leather and the studs will be plastic, and they will generally not have been riveted on very well.

If you are concerned over the legitimacy of a brand, be wary of where you buy the product from or google the belt and see if you get similar products in a shop or something.