If you’re shopping online, check to see if the belt label says “real leather” or “genuine leather.”
If a site doesn’t specifically say that, then it is likely legally prohibited from doing so because the belt is made of faux leather material.
If you’re buying it in the store, check to see if there are imperfections, wrinkles, or any other markings that indicate that this used to be an animal’s skin.
Also, check the edges — leather belts tend to have rough and/or uneven edges. Leather also has a specific smell. If it smells like leather versus, say, plastic, you’ve got the genuine article on your hands.