On Sun, 07 Sep 2014 03:02:32 -0400, Bassem Medawar
sharpened a crayon and wrote:

Are there ways to date overprint on a stamp without damaging the stamp?
I need to be able to tell if the overprint on a stamp is of the same
age as the stamp, going back say 80 years ago.

Age is a 'destructive' process, needing a sample of the overprint's
ink. If you really know your overprint, then visual differences under
a stereo microscope could become apparent.

Gloss, colour, quality of the strike, etc. There should be sublte
differences between the real font used, letter spacing, etc. and the
imitation.

On -=used=- items, it's a bit simpler. The stereo microscope will show
you whether the overprint is under or on top of the cancel. Most of
the GB forgeries used real stamps, from real postings, but of course
with the overprint applied later, it rested above the cancel's inking.