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.
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.The two sets of Dia-Compe brake lever bodies I checked had the four-number date code stamped inside the lever body (I couldn't find any markings on the levers themselves).However, a Gran Compe set of calipers had no markings. For example, 1182 means the 11th week of the year 1982.)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.