Don't think they'd cut MDMA with fentanyl. That doesn't really make sense. Expensive and would give you a completely different high that nobody buying MDMA would want. Usually they cut it with caffeine or other things that are insanely cheap.
You can buy a test kit online for $20.
https://dancesafe.org/product/mecke-...t-testing-kit/
I have one. I'm in Southern California and I buy most of my stuff from friends I've known for a long time at desert/forest type underground festivals, but even when I don't like 90% of the stuff I've tested has been legit MDMA. Surprisingly pills have been 100%. Only thing that really tested bad is the rocky type MDMA people try to sell as moon rock or " pure crystal MDMA " or whatever. That almost always ends up being some sort of bath salt/methamphetamine.
But yeah if you try to get molly from a stranger at a main stream rave or coachella type event you're probably not getting MDMA.