Determining Cardinal sex is very hard. Most people cannot tell the difference until after they have bred with each other.
One way to tell is to take a very very very close look at the anal vents of both of your fish.
The male with have too little v's pointing down and will look sort of like this ... v--v.
The female will only have one.. --v--.
These are very hard to see and you may need a magnifying glass.
Additional indicators that can help are: The female has more of a u shaped jawline while the male has a V like that rounds out at the bottom jawline.
The second dorsal fin of the male is much longer and usually reaches back in a flag like manner and at least reaches to the end of the tail. The female's won't reach that far. Also I have found many of the male's dorsal fins have a v like break to them.
http://www.marinebreeder.org/phpbb/v...p?f=166&t=2064
http://www.reefs.org/library/talklog..._102499/3.html
http://www.reefcentral.com/forums/sh...1659511&page=5
http://www.marinebreeder.org/phpbb/v...p?f=166&t=1295