With a pair look for the bigger one. When not in a pair it becomes slightly more challenging. If it is a large specimen for the species and let's throw out, over 2 years old, chances are it is a female.
Clowns are born sexless, for lack of a better term.
--Then they become males
--Then the dominant male in a group will turn female.
--If said female is removed from the group the next male in line will change to female, etc...