Serial number is ONLY valid method-BUT; for a quick look at one in a display case/show table:
a)checkered cyl. latches on pre wars(smooth on postwar)
b)checkered triggers and hammers on SOME pre wars(grooved on post wars)
c)2 piece cylinder retaining pin on prewar frame(1 lg. screw on post wars)
d)walnut stocks on pre wars,plastic Coltwood from 1946-54,or so,then wood again.
e)square butts,up to 1933,"standard",then round butts-BUT many exceptions,like square butts for the thousands made for military in W.W. 2(and sometimes found today with replacemeny bbls.)
I mentioned the sights in earlier post.
Probably more,but also remember,the 1946-1950 period was a transition period(as it was for S & W),so guns will sometimes show a "blend of pre and post war parts".