How can you tell if a toadstool is poisonous?

Check for white gills beneath the mushroom cup.

Turn the mushroom upside down, and check the color of the gills. Amanita mushrooms usually have white or very pale gills, which is an easy way to distinguish them as poisonous from other mushrooms.

Are toadstools poisonous to humans?

mushroom poisoning, also called toadstool poisoning, toxic, sometimes fatal, effect of eating poisonous mushrooms (toadstools). There are some 70 to 80 species of mushrooms that are poisonous to humans; many of them contain toxic alkaloids (muscarine, agaricine, phalline).