I am no expert, but once I got an old card back from SGC that said evidence of trimming.
I couldn't see it, so I took some photos under the microscope of the questionable edge, and then one of the "normal" edges, and I could, in fact, see a clear difference in the cut right on the edge.
That is in no way scientific, and I have no idea if it works all the time, but perhaps you could try that?
My microscope was just one of those ten dollar ones you get on ebay, but it worked really well with my cell phone camera.
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