These are great fish, i got a group of 4 and only 1 lived, but this one is great and has a peronality unlike all my tetras. They are also called sunset platys which i think is more common then the sunburst name. But anyways you can sex them by looking were there anal fin is, males have a short tube called a gonopodium in place of the anal fin. And the females have a regualr triangular fin there. Also males are usually smaller then the females by a good deal so im sure it wouldnt be too hard to size them up unless they are still relatively small.
Good luck!
- depthC