Posted By:
Matt"How do you tell the difference for future reference?"
The easiest answer to this question is to compare the card to an authentic example. If you can't have them both in hand, then compare their images online. Use ebay as a resource for finding the cards in PSA or SGC cases, then check the fronts and backs for major differences in color toning.
If you want a more accurate answer, David Cycleback wrote a fantastic article (available free online) about detecting the authenticity of early baseball cards.