I know that Trek have archives on their website going back to 2003 for all the bikes that they have produced, and providing the owner has not resprayed the bike then you can compare the colour schemes etc and what the original fittings should be. One problem though is that most bikes are modded by their owners, new bars, wheels, pedals, drive train saddle, seat post etc or have had parts replaced due to wear and tear, usually the drive train etc. In the case of a mod, then the parts may be better than the original, but if its wear and tear, its possible the parts may be substandard to the original as the owner could not afford to replace the original parts.
As what the poster before me says, he's basically right.