I've gotten some questions lately about this so I thought I would post a guide.
Let's start with the 'basic' postwar fakes. These are made on gray chipboard and on close inspection are 2nd generation images. Always stars. Here is a fake Musial:
The stock and splotchy background are the tells. Here is that card's back next to a legitimate one:
Don't confuse the Canadian cards with fakes. The stock is gray chipboard but the Canadian baseball cards that simply reprint the US baseball cards have either small diamond numbers at the bottom:
or large numbers:
