Question: How Can I Tell if My Red Eared Slider is a Male or Female?
There are fairly distinct differences between the male and female red eared slider, but they may not be obvious until the turtle reaches sexual maturity. For males, this is about they time they reach 4 inches in length (about 2-5 years old). Females are sexually mature when they reach 6-7 inches in length (may take 5-7 years).
Answer: Male red eared sliders have much longer claws on their front feet than females. As well, the tail of males is longer and thicker. The cloaca is located farther from the body in males, and males also may have a slightly concave (curved inward) plastron.
A good example of the differences in the claw and tail can be found on this page: Basic Turtle Information