Hi Andrew,
Male frog eyes will develop hemipenal bulges upon reaching sexual maturity. Males will also have darker yellowish scales between their back legs directly in front of the vent. They will almost look like stuck shed against the clean white scales on the rest of the stomach. If you don't know already, be VERY careful when handling them. They will drop their tails very easily and their skin is very thin. Have fun and good luck!

Zach