Glyn is right, of course. There's also the issue with tobacco cards of plastic sheets from the late 70s and early 80s that were slightly too small for some of the oversized T-cards, and thus owners actually trimmed their cards to fit them into the sheets. No deception was involved, but a card that was trimmed for such a reason would likely be rejected as trimmed.
Virtually every type of card, and every manufacturer, is different. Frank is correct in that there are telltale signs along the edge of a card that can help someone identify whether an edge was created with a cutting die, a paper cutter, a hobby knife or some other implement. People who have looked at thousands and thousands of cards learn how the edges of certain issues should appear when they've been cut naturally, and when an edge does not appear the way it should, there's a likelihood that the card has been trimmed.
-Al
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