First off the males once they get up to about ten pounds will have a distinct humped back or shoulders is what some people call them and they also have a rounded head or more muscular head than the females.
The Female Bluecat is more of a streamlined fish with straight gill plates and not muscular looking at all. It doesn't matter how big a female gets either. They'll never get the humps on their backs between the dorsal fin and the head on both sides like the males have.