Vintage synths, guitars, amps, stompboxes, potentiometers, speakers and many other components made in the US often have an EIA code on them which will identify the manufacturer and the date of manufacture. Gather together all the date codes from those components and this tool will tell you that the unit was made some time after the latest date (not including any replacements of course).
For pots this code is 6 or 7 digits. 6 digit codes may not be able to be narrowed down to a specific decade as only the last year digit was included into the number, some codes may return more than one option. The codes may contain spaces or dashes, if so please include them. Pots made by IRC use their own format which is also supported, this will start with 615 and will have 2 spaces in it, you must include those spaces so that the this tool knows its an IRC code. Many components also contain product codes and specifications so unless the manufacturer comes out correct you should not rely on this method of dating alone. If the manufacturer matches what is on the component it is safe to assume the date calculated will be correct.
It should be noted that large manufacturers often stocked up on components in large batches so the unit may be made a year or more after some of its components. In 1966 Fender bought a huge stock of pots which were used right up until 1971 so it is possible for a 1971 Fender guitar to contain 1966 pots. I have also included some Gibson guitar specific part codes in my database which will be automatically detected and decoded.