The sticker doesn't peel off cleanly; parts of it get left behind in a distinctive checkerboard pattern.

There's an example at this link: http://www.psacard.com/Services/PhotoProcess

I do not own any PSA/DNA, or other certified/authenticated signatures yet that have so-called tamper proof stickers, but how on earth do they work? How can you tell that somebody has not simply peeled it away and put the sticker on a photocopy of the original autograph?

I'm aware that apparently PSA's stickers get ruined once you try to tamper with it, but...