Also be aware of the following:
1. The 1948 HOF set was reprinted by the company in the 1970s on thinner, whiter card stock. It is genuine but worth only a fraction of the original.
2. Some of the cards were printed in Canada in the 1950s with actual card numbers on their fronts on a much coarser gray stock.
3. A series of 4 on 1 cards was made by Jim Rowe in the 1970s and are basically a Broder set with very little value but are occasionally offered as the real deal from the 1920s and 1930s. Look at the Standard Catalog for a list of them.
4. Some of the 1950s cards were made into a border trim wallpaper.
5. Sometimes you will find the cards printed on shiny paper stock. Those are not fakes, they are advertising pieces designed to be pasted onto the placards that went into the machines.
6. In terms of fakes, I've noticed three different varieties: gray back, white back and a very coarse gray back stock. Any genuine card will have a smooth, creamy stock back, unless it is a Canadian print, and those will have card numbers on them.