[Eric72]

One useful tool for detecting added color is a black light. There are numerous inexpensive portable models available online. I paid roughly $20 for mine on Amazon about a year ago.

Regarding the edges of '71 Topps baseball cards, you might also want to try this:

After looking at both the front and back of the card, take a direct look at the edges. If the card has been recolored along the edges, it's very likely you'll be able to see black ink has bled into the edge of the card.