I collect mainly stills from the 20's and 30's so it's fairly easy to tell reprints of those. The paper is different, reprints are usually a lot glossier, the weight of the print may be different, and since the the negatives don't usually exist for these, they are usually printed from copy negatives (someone who shot a picture of the original still and makes a negative from that to make prints) which can be a bit softer and are usually cropped. Snipes (the little paper on the back) are usually a good indicator the still is original, I haven't seen too many fakes of those but they are usually pretty easy to notice as the paper they usually used was a cheap, course paper, not like the ones you would put in your inkjet. I also have many photos which have handwritten dates and stamps on them.
Once you get into the 40's and beyond, you're on your own.