Buy from knowledgeable sellers.
For T206s, Goudeys, get a few cheap commons first. Reprints and fakes will be distinctly different than the real cards.
Learn to identify printing processes, so you can tell both antique and modern printing. This is of particular help when you are looking at a card you are unfamiliar with.
Ask for second opinions, such as from people on this board.
If a card or other paper/card ephemera smells musty, that's a good sign.
A year ago I came out with a new guide to antique printing processes and authentication of prints. It is a general printing guide, but covers the processes for cards and for those collecting other items like programs, signs, etc. Baseball cards are prints, just little ones on cardboard. The guide is in pdf and well illustrated. It's concise, but a pretty complete guide for the area and I wrote it in part so I wouldn't have to write about it anymore.
printing guide