This is usually true, but you can't be sure with this method. I have a breeding pair of carpintis and the female doesn't have a black blotch on it's dorsal fin.
tru dat fishguy7! Im sure everyone is tired of me saying this, but it seems more often people are mistaking females for males. this pic is of a female, no traditional female markings, no way to tell until she layed eggs. If you buy some, dont be so fast to call it male.