male or female turtles

Question
how can i tell if i have 2 male or 1 of each or 2 female.
as the last to days the bigger 1 of the 2 has been stroking the other ones face with its front claws. and have been told that it is a sign of mating?
many thanks

Answer
Different species have different signs. The main ones are:

Male: longer, thinner tail with the vent in about the middle. In-curved belly shell (plastron).

Female: shorter, wider tail with the vent near the base. Flat or slightly bulging plastron.

For the most part, most of these signs won't show up until the turtles hit sexual maturity. For many species, this is about 4" long.

Besides the main signs, in sliders (like the Red-ear slider), the front claws of the male are especially long and used in mating rituals.