Instructions
Look at the tail end of the plastron, or bottom shell, of the tortoise. On males, there is a concave curve in the shell, whereas females typically have a flat plastron. Do not flip the tortoise over to check this; instead, gently raise the animal and look from below.
Check for a long gular. The gular is a part of the plastron that extends underneath the neck. Males develop a longer gular to help them while fighting. On females, the gular does not reach much past the rest of the lower shell. However, males' gulars may break off in combat, so this can be a misleading form of identification.
Compare tail lengths. Males have a longer tail than females, but both tails are very short, making this method of identification a little tricky.
Notice if the tortoise has enlarged chin glands --- look for two visible sacs on either side of the chin. Females almost never show their chin glands, whereas they are often visible on males, particularly during mating season.