You just have to know what to look for. Different issues have various tell-tale signs. Some general clues:
(1) The type of striations or rough-cut on each cross section. Look for uniformity. Generally, but not always, a smooth edge will indicate trimming (exceptions: vending and presentation cards and certain T, E, and W issues, among others).
(2) Sharp edges. If a card appears to be recently removed from a pack, it might very well have sharp edges; but a card that has some surface wear or toning, generally will (should) not.
(3) Size. Some issues have a great deal of size variation and different types of production methods often yielded various dimensions. For the most part, this is the least reliable indicator, but one that you should still be aware of.
(4) A bowed edge or "bat wings." Sometimes when cards are trimmed they exhibit an edge which is not completely straight. Also, "extra" material near one or more of the corners (front or back), may indicate trimming.