My late grandmother had an infallible method for determining a mushroom toxicity.

Her sister and brother-in-law would go out into the forest preserve to collect mushrooms. They would alway divide a portion for her and drop them off at her house on the way home.

She would then place the mushrooms in paper bag and store in a dark cool pantry overnight.

The next day she would phone her sister and ask, "How are you feeling?"

If they didn't get sick then she figured it was safe to eat them!