It's very difficult to tell the difference between a male and a female banggai cardinal until you've watch them breeding and then, it's easy. The distinguishing difference between the two is that the male's top front fin is a bit longer than the female's.
In this picture, the male would be the one in the front:
<img src="http://www.erdingtonaquatics.com/imagemarine/banggai6.jpg">
We had no idea which was which when we got our Banggai pair, but after seeing which one had the eggs in his mouth, we can identify them now at just a glance.