Hold them up to/near a light source (EDIT: So you can still see the card surface, lol), and move them around a little, and see if there's a "rainbow" like reflection that comes back. EDIT AGAIN: Depending on the colours of the card, it's usually in the form of oranges, yellows, reds, greens, purples, etc. (bit like a rainbow, ha ha).
I don't have any on me, but I can do a scan later of a couple, else jump on ebay and search for refractor.
Else, post scans/pics of what you've got, and we'll let ya know!!