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hankronFor collectors, the best thing is to gain a good eye for the issue through handling and observing, and by getting good second opinions. The human eye is a sophisticated tool in all areas of collecting. A favorite quality grader would be an example of a good second (I didn't say final) opinion. Buying from a highly reputable dealer is another example of getting a second opnion. Black lights are at best of minimal use for detecting trimming-- which doesn't mean you can't experiment yourself with a black light ... One can also skip the Gem Mint type of stuff, and collect mid to lower grade cards. With lower grade cards, trimming is less likely, usually more obvious and less detrimental to the card's value ... There are advanced techniques for judging whether not a card is trimmed, but they are neither practical or inexpensive.
I'm also a firm beleiver in the significance of provenance as helpful in judging items. If someone is offering you an expensive Mint card, ask him to prove to you where he got it. The difference between a copy of a Lew Lipset invoice and "I bought it in a Yahoo auction" should be obvious.