The following are not clean-cut rules, and not all work for all
turtles, but using a combination of them, will usually help
you determine the gender of your turtle.
* Males often have fatter, bigger tails than females.
* Males have the vent (cloaca) about 2/3 from the shell towards
the tip of the tail. Females have it closer to the shell.
Other, less reliable methods:
* Males have a flat or concave plastron (bottom shell) - so it will
fit better on top of the female. Females have a flat or convex
plastron - so there is more space for eggs.
* Male sliders grow long claws on their front legs.
It is pretty much impossible to tell the gender of a baby turtle.
Since all of these are secondary gender indicators, they develop as
a turtle grows up. How soon you will be able to tell, depends on the
size and age of the turtle; I can't really tell you when, but it may
be a couple of years.