Of course, all this assumes the bike has the original component.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.
For example, the back of cranks are usually marked with crank arm length in mm, typically in the range of 165 to 185.
Seatposts are marked with outside diameter, also in mm.
Especially useful, and challenging to sort out and verify, was the Shimano code.
Also gathered on this page are date codes decoded and generously provided by others.
Unfortunately, many of these are coded, and require some additional knowledge to understand the code.
If you know of other components that are marked or coded that can be added to this list, please let me know.: The information on this page is copyrighted.
Following Ben's lead - I checked three sets of Dia-Compe G calipers and all have the four-digit date code on the back of one of the arms.)Most early Treks (1976 through about 1980) were sold as framesets.The components were added by the local bike shop or by the buyer. Components also could be swapped from an existing ride to the new frameset.As the wear on a vintage bike increases, the greater the likelihood components are not original, either through replacement of worn parts or through component swaps.Swaps can take place early in the life of a bike as the proud new owner upgrades to new or used components of higher quality.