Check the label on the couch. Imitation leather goes by different names, including Naugahyde, Glaze Leather and Symphony Suede.
Look also for labels that read "bonded leather." This type of couch is made out of leather scraps that are glued to a fabric backing, making it only semi-authentic and inexpensive.
Consider the price of the couch -- if it costs less than $1,000, it's likely a faux leather or bonded leather.
Know your grains. For example, "Top grain" leather is a misnomer -- it is not, in fact, a particularly high quality grain. "Top grain" refers to leather with an imitation grain that is stamped onto the leather after the real grain is sanded off.