Refer to this thread for detailed info on how to tell.
Hollow Sounding Door
What I finally learned was that my passenger front door was not original, there was no VIN number stamped onto it. You will find the VIN # on every peice of sheet metal. They are on the undersides of the doors, are on the bottoms of the bumpers, when the trunk is open, you can see that one... and the 2 rear quarter panel's VIN are seen when the trunk is open (same idea with front quarter panels when hood is open).
My car has been repainted and the only peice not original is the passenger door. Oh well, other than that, car has been just great. Be sure and review the afforementioned thread!