If at all reasonably possible, try to find a place where drugs can be tested to see what they really are (sadly a rare thing still, but it exists).

The next best thing to do is talk with people in the scene and find out what they consider a trustworthy source. Any single person trying to offer you pills is a potential scammer or worse, you need to verify from other sources whether they're legit.

There's probably a better than 50% chance you'll get something close enough to what you want regardless, because incentives are aligned with everyone having a good time. But just like everywhere else, bad actors will always exist.