What assurance do I have that some rando at a festival who claims to have "magic mushrooms" is actually selling psylocybin mushrooms? None.
They might have picked some "little brown mushrooms" in a field somewhere thinking they were psylocybin mushrooms but not gone to the effort to identify them properly, or just not known how to do it, or made a mistake. Or maybe they got it from someone who told them that's what they were, but themselves didn't know, etc.
Or they (or their supplier) might have gone the easy route and bought some mushrooms and sprinkled some "bath salts" on them.
As a consumer you just don't know -- especially if they're dried, crumbled mushrooms which could be just about anything or a mix of stuff.
It's just sad hearing people who are so adamantly determined to stick their head in the sand and trust whatever drug they're being peddled is what their dealer claims it is.
Nancy Reagan's famous line was "Just say no." I'm not saying "Just say no". I'm saying "Just say know."
Educate yourself. Test your stuff. Be smart. Don't just guzzle down whatever you're given without satisfying yourself that it actually is what your provider claims it is.