Gas chromatography can detect relative percentages of a pill's constituents, of course, but that's hardly something you can do at home, never mind at a party or club, and even if you sent a pill to a lab, AFAIK they're not allowed to release strength information, so you wouldn't even know after the fact.
If you could somehow get the MDMA separated from the binders and fillers in a pill, you could then weigh it to get an idea how much you had. I don't know if that's possible, though, and it's certainly not something you could do on the spot, either.
Summary, there's just plain no way to field-test for strength that I can think of.
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Live wrong and preposterously.