Behavior mostly. If a clown is alone, is a female. If a clown is trying to kill another clown, the aggressive one is a female.
Almost always, the big one is the female.
If you place a larger clown and a smaller clown together, you sort of nudge them in the direction of which one will be male and which one female. But this does not always work. Just today at the shop, the owner placed a tiny new clown from a tank of them with a larger female, and the female did her best to kill it (they are both percs). Seems the smaller one must have already started the change. I had to separate.
Always a good idea to have a net handy when pairing clowns for the first time. Often is apparent immediately that the female won't accept the male. Sometimes takes a few hours.