Posted By:
barrysloateTed- SGC published a detailed article about card trimming in their recent collector magazine, and measurement is just one test they use. They also study the edges of the card, to see if they appear to be original. A card cut in the factory will have a kind of convex edge (if that's the right term) that can't be duplicated by an amateur. While I found the article fascinating, I'm not sure I could identify an original edge every time. You need good magnification, good eyesight, and some practice.
edited to add if a card is already in a holder, you can no longer see the edges and would have to break it out.