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.
I have a set of Dia-Compe mountain levers where if you pull the lever all the way, a piece of the lever is exposed, which has a clock-type date code.
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.
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.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.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.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.