From the WetWebCrew....."" External --- differences are very difficult to determine. For the Centropyge in general, some people think that the males typically will have more pointed anal fins than females, others will tell you that the colors are "sharper" in males...In he end, the behaviors will be a better clue, IMO. Do read up on this fish and others in the group on fishbase.org, or look into Centropyge breeding specialist Frank Baensch's work on the net...The material is out there-you'll just have to do a little digging ""...
Best to get several juveniles because you may know the female can change to male but not reversed. And a male and several females can coexist if given enough room but two males will fight.