Look for the whitish stalk that is about 6 inches (15 cm) long, with a large, round bulb and white, sac-like volva, a remnant of the tissue that protected the mushroom's gills while it grew, at its base.
Measure the cap of the mushroom and look for a green or yellow color. The cap should be about 2.25 to 6 inches (6 to 15 cm) wide and can be olive green, pale green, or yellow, and sometimes white or brown, with 1 or more patches of thin white veil tissue.
Dig down a bit into the soil to find the lower part of the mushroom's stalk. The lower part of the mushroom's stalk, including the bulb and volva, is often buried in the soil around the tree it is attached to. The bulb may also break away or fall apart over time, so if it isn't present, the mushroom could still be an Amanita mushroom.
Look at the flat, wave-like edge of the cap. The cap is convex in younger specimens, but flattens as the mushroom ages, developing a wave-like edge.
Look for many crowded, white gills underneath the cap. The Death Cap, and other Amanita mushrooms, display white gills on the underside of the cap that are densely crowded together but do not extend all the way to the stem. The gill color is another way to distinguish a Death Cap mushroom from the Paddy Straw mushroom and other edible mushrooms. The Paddy Straw's gills are pinkish brown. Other mushrooms, such as those of the genus Agaricus, also have pink gills, although theirs later turn brown.
Look for a white spore print by placing the mushroom cap on a piece of paper with the gills facing downward and leaving it overnight. A Death Cap mushroom will leave a white spore print, while a Paddy Straw will leave a pink print.
Smell the mushroom's flesh. A Death Cap mushroom smells slightly like rose petals; this test can be used if you can't tell from its physical appearance whether the mushroom is a Death Cap or other variety.