A simple way to distinguish leather from vinyl is to inspect the grain of the material; vinyl has a distinguishable grain pattern, while the grain in leather has no discernible pattern. Vinyl contains a pattern because it is manufactured. The hide of an animal is never the same, making it impossible for leather to have a pattern.
Another way to tell leather from vinyl is to smell the fabric. Leather is treated with chemicals and dyes when it is processed, giving it a distinctive odor that vinyl doesn't have.
Examining the item's tag can also give insight as to what kind of fabric it is made from. Leather is noted as "genuine leather," while vinyl is often labeled as "pleather," "faux leather" or "bonded leather."