The quikest way is to look at the anal and dorsal fins. In the male they will usually sport more pointed fins and in the female they will be more rounded. Also, depending how many you have in your tank you can always watch for the alpha male. He will be the one with the most territory and the most aggressive. The most sure fire way is to wait until they begin to breed, however i have seen my female lay the eggs but i knew what she was before the start of that. You can sometimes tell by the fins when they are young, But it is rare so pay attention as they grow. If anyone has other suggestions or wants to contradict me. please go ahead. This is the way iwas taught. hope it helps