> There is no easy way of knowing what is inside a tablet

Chemical tests.

> but the “pure” crystallized products are easy to recognize mdma especially

Lots of the extended amphetamine family (including the various -drones) share very similar tastes, and crystallise very similarly. If you wash MDMA that contains these other substances you'll end up recrystallising those as well.

> And it would have to be a damn big tablet to contain too much mdma... by bet is on what else is inside.

Then your bet is factually wrong and likely dangerous.

Test your stuff. Know what you're getting, start low, and don't listen to people who 'know what the real stuff looks like'