Asked in Turtles and Tortoises
How can people tell if it is a female turtle or a male turtle?
This is highly dependent on which group the turtles belong to.
Some turtles and tortoises, but not all of them by far, have differences in the curvature of their plastron (or "belly"). A male would have a plastron that is concave (bending inward), while a female would not.
Some turtles have secondary sexual characteristics: In adult male Deirochelyinae turtles (Painteds, Sliders, Cooters, Map Turtles), the males have comparatively much longer claws on the front feet than females of the same species. In many species of box turtle, an adult male will have a different colored eye than the female, although which color this is changes between species.
Size can be used, as in many species the females are larger than males (some species significantly so to where males have a different diet), but to use this one must have a relative guess as to the age of the turtle.
One of the best determiners is the tail. Males often have a larger and longer tail with a cloaca that is farther from the shell, while females often have short stubby tails. While in some turtle groups this is a large enough difference to give the male a massive tail, in others an unexperienced eye would need to compare to a known male and female of the same species to be sure.
In a very large percentage of turtle species there are no chromosomal differences between males and females, as the hormones controlling gender are instead determined by egg incubation temperature.