[the 'stache]

Ted, the tools already mentioned will all be invaluable in helping you determine if a card has been altered (not just trimmed), but nothing is going to help you more than just handling the cards you want to collect, and looking at them up close.

Say, for example, that you wanted to buy a T206 Christy Mathewson portrait in very good condition. You're at a vintage card show, and you find a nice example with brilliant color, good centering, and nice clean edges. You want to know if they've been trimmed at all. You really want to have done your "homework" before ever going to that show. Start out buying commons from the set. Ungraded ones. Pull out your loupe, and look at them up close. Feel them in your hands. Smell them. Observe what natural wear over time looks like. Get a really poor T206 common. Take your ruler (I have used a metal drafting ruler), and an x-acto knife, and cut a new edge. Look up close at that edge under magnification. See how it looks, and how it differs from other edges of cards you've bought. When you have examples like these, and have seen them up close, and under magnification, you're going to be able to identify what a factory cut looks like, and what an unnatural clean edge looks like.