Usaully by 5-6" you can sex them accurately. Males will have spots on their face, and upper and lower body. They'll also have more distinct markings on their fins and tail. Once you see lots of them, you can also tell by their body shape most of the time. Females often have a more golden base coloration and splotches down the center of their body but without the facial and body spots and less distinct fin and tail markings.