Adult males have longer more pointed dorsal (top) and anal (bottom) fins. They also have a lot more red around the throat.
Adult females have shorter more rounded dorsal and anal fins and not a lot of red around the throat.
However, poor quality males don't have much red and can have short round fins.
Juveniles are hard to sex. Look for longer pointed fins, which should be males, and if possible, get a group of 8-10 young fish and grow them up together. Let them pick their own mates.