Posted By:
RobertSScott--
It's just like cards -- if it doesn't feel right, it isn't. Thickness is a dead give away, as is printing that appears newly stamped on the back not printed.
If a card is too thin and seems like it wouldn't make it through the mail, it probably is a fake. These things were made to be tough enough to withstand the US Postal Service! (Some were thick, some were thin, but none were paper-like thickness.)
Similarly, if the printing seems to be recently stamped on the back, not professionally printed, it might be a recent fake. Not always the case, but sometimes. Of course, laser printing is also a giveaway (if the paper is "aged" but the printing is shiny when reviewed on an angle) -- just like the fake Kendig baseball candy cards floating around a few years ago or the Yankee Stadium/Ruth postcard in this thread.
Here are the cheat sheets I use for stamp box and stamp backs:
Stamp Boxes (e.g. AZO, Cyko):
http://www.playle.com/realphoto/photoa.htm
Stamp Backs:
http://www.playle.com/realphoto/backa.htm
And I'm sure David Rudd could probably add to this by stressing what to look for under microscope.