you can not tell by looking at them , SOME but not all males show a higher dorsal fin , as not all males show this then it is of little use , on a fully grown fish venting is possable but channa venting is much harder to see than say a malawi cichlid and a jewelers lupe is often needed and a very helpfull fish .
the other problem is that just putting together a male and female will not mean that they will breed , channa form a bonded pair from a group and bond for life in the aquarium , if just putting a male and female together it is very likley they will just kill each other .
cheers col