Pull the lever and look inside the top of the lever arm for a code such as "1084." Dia-Compe extension levers (yuck) also tend to have date codes on the side that faces the brake hood.
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 rear derailleur freewheel/cassette and chainwheels are probably the first to be changed on a bike.
On a vintage bike in excellent condition (that apparently had a lonely existence in a garage) all of the components likely are original.
For old Treks, 27.2 is the most common, but for other bikes the diameters can range from 25mm to 33mm.
The rear derailleur can often be dated to a year or two by referring to the book "The Dancing Chain - History and Development of the Derailleur Bicycle", by Frank Berto, 3rd edition 2009.
The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.