The only reliable way is by the shape of their breeding tubes, which are only visible when they are paired and laying eggs. Supposedly you can also tell by examining their head for some kind of 'bump' or indentation, which would be a male. Smooth head would be female. However, while that method sounds easy enough...it isn't always.
Sorry yeti, One is under the thread "male breeding tube" and the other is in the thread about " I got eggs lot of them" just scroll down and you will see my female. Her tube is more rounded at the end and the males is smaller and more pointed. Hope this helps..
Breeding ANgels is fun but can be frustrating too.. I have had many batches go don the tubes on me. So far to date have only gotten to freeswimming and then they go poof and I have to start over. Wish you lots of luck if you dI it is really cool to see them breed