Wax...Or Gum?
 

You know, looking at them closely, quite a few of my old cards have stains on the back. Honestly I can't tell the difference between what would be caused by wax from the wrapper or by the bubble gum. None of them are really noticeable, but rather pretty faint except for a couple of cards. Any ways to tell the difference if the stains are faint?

Shape, color, location, and whether they bleed into the card stock.

Its been a long time, but I seem to remember a lot of the stained cards I experienced were wax. Newer cards fresh out of the pack might have been affected by the gum sometimes, I don't recall, but I have seen some really old packs and the gum staining is nasty, sometimes even so bad the gum won't even come off the card. I assume that is from the ingredients in the gum breaking down over time.