Males generally have rougher backs and their forearms are thicker than the females... but they're almost identical... Except during  breeding season, when males get black horny  pads on their fingers and forearms... Another way to try and tell is to observe their behavior... When a toad tries to jump on the back of another toad and use the arms to grasp it, it's almost always a male.  
It's not that easy to get fire-bellied toads to breed, so I wouldn't worry about separating them even if they are male and female... If they do happen to breed you can just remove the eggs once they're laid.... If they don't eat them before get the chance. ; )