Hello there, the only reliable way to tell if they are male and female is by looking at the plastron(bottom shell), upside down, in profile. You can do this by putting each Tortoise on its' back and looking at the plastron from the side. If the plastron looks sunken in, then it's a male. If it looks straight and even, it's a female. By the way, if you want to sell them, please let me know.

Good luck and take care, Johnny.