Photograph as much of the details of the removal of parts. Many times you can see that a part was always on the car. Like if a seat nut has the OEM undercoat or OEM paint on it, most likely it is original to the car.
If push comes to shove you could make your own fender tag. You can get reverse letter punches on ebay.
Disclaimer... I am not suggesting you make a fender tag to make your car more valuable by faking desirable options. If it started life as a \6 it should have the fender tag for a \6 even if you swapped in a V8.