Buy a field guide for mushrooms, or borrow one from your local library. Choose a book that has illustrations of Chlorophyllum molybdites and Macrolepiota rhacodes. Also, search for illustrations of these mushrooms on the internet. Obtain mature specimens of these two mushrooms. The biology department of a high school or university might have specimens that you could examine, or you might find them in botanical gardens.
Examine your specimens and observe the pictures of the two mushrooms in the field guide and on the internet. Note that both have a white or cream-colored cap mottled with brownish flecks. Note also that both species are tall mushrooms with a large cap about the same size. Because of these factors, the external appearance of the mushroom does not help you much in your efforts to distinguish between them.
Examine the stems of the specimens. Note that both have rings that can be moved up and down the stem. Often the presence or absence of a stem helps in mushroom identification. However, since both species have movable rings, they offer no help in distinguishing Macrolepiota rhacodes from Chlorophyllum molybdites.
Examine the gills of the specimens. Notice that both are whitish in color. If you have an aged specimen of Chlorophyllum molybdites, note that the gills have assumed a greenish color. These greenish gills show that the mushroom is not Macrolepiota rhacodes. Both Chlorophyllum molybdites and Macrolepiota rhacodes tend to grow in circular areas called fairy rings. If some members of these fairy rings have greenish gills, chances are that all the mushrooms in the ring are Chlorophyllum molybdites. However, the presence of whitish gills is inconclusive, since both Macrolepiota rhacodes and younger specimens of Chlorophyllum molybdites have them.
Obtain spore prints of the two specimens in the following way. Take two sheets of white typing paper and place them on a level surface. Place the cap of the Macrolepiota rhacodes specimen on one of the sheets with the gills downward toward the paper. Cover the mushroom and the paper with a bowl. Do the same thing with the Chlorophyllum molybdites specimen, using another sheet of paper and another bowl.. Do not disturb them for three or four hours.
Remove the bowls and the mushroom caps from the pieces of paper. Examine the area of the paper that lay beneath the mushroom caps. Note that green spores have fallen on the paper occupied by Chlorophyllum molybdites, and whitish spores have fallen on the paper occupied by Macrolepiota rhacodes. This difference in spore color distinguishes Chlorophyllum molybdites from Macrolepiota rhacodes.