While not a perfect indicator, with Sigs there should be a grey or blue band on it(if it was not a display model). Other than that, it may or may not have some packing oil still on it, also depending if it was a display gun or not. You can look for wear on the rails and barrel also. But it would be tough to tell no matter what if a part was swapped out without knowing sig parts intimately and detail stripping it.
Do you have a reason to believe you may not be getting what you paid for?