There are services that accept small samples by mail (from anywhere) which they then test for a small fee and email you results. You can also buy reagent test kits online, though they are a bit less accurate than what the testing services use.

There are also very simple methods for "washing" what you buy to increase purity and thus dose more accurately + reduce side effects.

In Amsterdam, you can test it for free. It's strictly speaking illegal, but in practice, the government doesn't prosecute these things as they try to attack it from the supply side and try to protect the consumer side.

So you buy some MDMA, take a bit from it, bring it to the lab, get it test for free, and presto! The only risk is financial risk.

Disclaimer: I've never done it, I've only see some (probably state-sponsored) videos on it.

MDMA is so cheap to manufacture anyway, and analogs that are cheaper are usually massively qualitatively different (of course if you’ve not experienced MDMA yourself before that doesn’t help much) — but it’s also easier than ever to send off some of your sample to be tested in a lab if you’re concerned. A benefit of that global market, too!

MDMA can be reagent tested. Still this is the game we play when these substances are illegal.