Usually you would call that "dating" the amp. When I clicked on this I thought I was going to give suggestions on how to make the amp look older.
Anyway, if you look at the PCB in the amp, it should tell you what revision it is, which can help with figuring out how old it is. To do that you would need to take it apart, but usually when getting a used amp, I always pull the chassis to check everything and clean the pots.