Instructions
Look at the length of the tail. Males generally have larger and longer tails than females. When fully developed, a male's tail will be approximately three times the size of a female's. The female has a tail with a short base, tapering down into an increasingly skinny point. Males tails are approximately one inch below the bottom of the plastron, which is the bony plate on the underside of the shell. Their cloacal opening, where they excrete waste from, is further from the base than females.
Evaluate the height of the shell. A female's shell has a higher dome on the carapace, or top portion of the shell. In comparison, males tend to have much flatter shells.
Look at the size of the head. Males have skinny necks and small heads, while females have large and bulky heads.
Evaluate the overall appearance and size. Fully mature female terrapins are larger than the males, and average six to nine inches long. Males are rarely longer than five inches. In general, the female appears bulky, as compared to the male that looks more thin and streamlined.