Ted,
When you have a pair, the larger one is the female. You can also see her lay eggs and/or have her ovipositer extended when it is time to lay eggs.
If you a clown by itself, you can sort of guess by size. They all start out male and the males tend to stay small. The size difference is really pronounced in maroons and deeper bodied clowns and less so in perculas and ocellaris.
Adam