well they are usually experienced, however yes there are some who may not know, even before that, do your research carefully first, learn how to differentiate between authentic and non-authentic..

when you are at the store, look at the authentic ones and try and compare them; or buy one from the retail store to gain deep knowledge on how to spot fakes, fakes nowadays are so advanced - sometimes the only way to tell authentic apart is only when you actually own one when you're familiar enough with it..