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'
It is unlikely most people will have access to equipment with the precision necessary at home. There is an organization out there (https://www.ecstasydata.org/) that does anonymous GCMS tablet tests of MDMA in particular for $40 (and other substances for a higher price).
The typical tests out there for most people are reagent tests -- chemicals known to react with a desired substance to turn a certain color. Reagent tests are not foolproof -- the main thing you know is that if you do not get the desired color, you most likely don't have the correct product. It is possible to fool a reagent test. They are better than nothing for sure.
Taste, color, and crystallization are not really reliable ways of identifying the product.
This is all very poor advice.
Get chemical test kits, use drug testing services where they are available, be cautious.
Which is pretty terrible itself, powders and crystals can be just as adulterated as pills. Relying on sight and taste to tell what you've got is very unreliable.