At first glance, male and female turtles appear very similar, but there are a few ways to help them apart. Unique sexual characteristics differ among species, but there are some traits that can generally help distinguish males from females. Many of these characteristics are relative to each other, so are more evident if you have both a male and female to compare.
Examine the plastron, or shell on the belly of the turtle. If it is concave -- caves inward -- the turtle likely is male. Males climb on the backs of females to mate, so having a shell that is curved in a bit makes for a better fit.
Examine the tail. Males' tails typically are longer and fatter than females' tails.
Examine the cloaca, the common opening for the digestive and reproductive systems. In the male, the cloaca is more toward the tip of the tail, and in females is closer to the plastron.
Examine the front claws. Males have longer front claws, which are used in attracting females for mating.
Look at the color on the face and forelegs, where males are said to have more bright orange and red markings.
Look at eye color. Males typically have orange or red eyes, while yellow or light brown eyes are common in females.
Consider overall size. While females can grow large, male turtles typically are considerably bigger. As stated earlier, this requires other turtles for comparison.