Yes they are Dimorphic, the thing is it can be tricky. Males typicly have more red and the red is brighter, the dorsal fin ends in long filliments that extend past the tail fin , and males usually are a bit bigger than females at maturity. These days though the differences are less distinct than in times past and you will hear about females that look better than a lot of males . IMO this is the result of the large scale breeding that is done to supply the hobby. Little if any quality assurance gives use fish with poor color and an androgenous apperance. A keen eye will be able to pic out the differences between the males and females though and venting (observing the difference in shape of the protruding reprodutive organs) is a 100% accurate way to sex them.