Numbers Matching
If you have a fender tag you can find the scheduled production date for the car. The engine will be stamped with the actual build date of the engine on the pad in front of the right head under the distributor. IF the engine build date is prior to the SPD you likely have the original engine, but it can even be after the SPD if the car build was delayed for some reason. So, you can never be absolutely certain, but you can be close.