That's why it is important to buy from a shop you frequent. Usually, the usual shops (CP, AP, MS, J3:16, Lords, TK, TCW, etc..) don't need you to prove it's a bad copy. If they have other copies, they will allow you to swap. It's more difficult if the lens is ordered and they only bring in one piece for you.
First you have to got to know that it is a bad copy. How do you know that you got a bad copy in the first place.
It would depend on the shop on whether they would ask you. I get my lenses from reputable shop like lords and CP, and I never have any problems with
the lenses.