You wont be able to sex one. Get another clown that is a little larger than the one you have now. The dominant clown will morph into the female, In a clownfish pair the larger of the two is the female. There is no way to gaurantee that you will even be able to pair your clown back up. Luckily you have one of the least aggresive clowns.
It looks like you have a An ocellaris, which is a false perc.