Spore print

A spore print of a mushroom is made by placing its spore-producing surface flat on a sheet of black or white paper, and placing a weight on top of the mushroom. The mushroom is then left overnight in this manner. When the weight and mushroom are removed, the color of the spores is visible, contrasted against the paper.

One rule of thumb for determining if a mushroom is poisonous is if the spore print is green. While not every poisonous mushroom has a green spore print (in fact, most don't), about 99.5% of mushrooms with green spore prints are poisonous.