[Anthony S.]

Tony,

One frequently seen indicator that a card is a reprint being passed off as an original card is the wear pattern at the very bottom center on back of the card. If you'll take a gander at the back of that Gehrig (use the enlarge option that Ebay provides), you'll see that right below the phrase "Indian Gum" the wear pattern is especially strong and deep. As if someone took a pencil eraser and erased vigorously on that one spot, which is likely what happened. The reason for that worn spot is that that was where the word "reprint" originally appeared on the card before some enterprising weasel erased it.

Now this isn't a foolproof method to detect fakes, as some fakes are photocopies of original cards, but in the case of cards from reprint sets being altered, the heavy erasure at the bottom middle on the back is a common indicator that you're being duped.

Edited to add scan of the bottom back of the Gehrig card in question: