Guess.
Some say the males have larger jaws but that is pretty subjective.
In fact mine turned out to be the reverse of what I always thought. I was convinced Sunshine was a female and not sure about Moonshine. Then Sunshine started flashing and the next thing I knew Moonshine swallowed a basketball.
I figured. Seems like it's almost impossible to id the gender of most fish. I wish I could tell if my tangs were male and female. I always call my yellow one a "him" and the blue one a "her" even though I have no clue if that's right. lol.