That one's easy! Learned this 30 years ago when I was trying to pick up mycology.
You don't hork the whole shroom down, just a tiny, tiny nibble. All good? Bigger nibble. Heartburn? Wonky guts? Stop. Tripping balls and seeing the ancestor's Dreamlands? Maybe a bigger nibble. You get the idea.
Won't take long until the tribe figures out what local fungi are safe or not.