Generally, if you introduce two clowns at the same time, and they're both different sizes, the larger will become the female (if she wasn't already), and the smaller will remain a male. You'll see some signs of which is the male by noticing which fish swims in front of the other and sort of vibrates. That's the male. Generally, once a fish has assumed a female sex in a breeding pair, she won't change back to male--but there is anecdotal evidence this has happened, I just wouldn't count on it. If you have two clowns that are the same size, let them work it out amongst themselves; one will grow into a female, and the other will exhibit the vibrating behavior mentioned above.