Answer:
There's a number of ways. Extractor marks on the brass from previous use, different coloured wax than what the original manufacturer uses, original headstamp grinded out for a new headstamp. Best way to know for sure is simply to know the source of your ammo. There are companies out there (such as Georgia Arms) who sell reloads exclusively. Order from them, and you know you're getting reloads on once fired brass.