I'm no expert on them, the only way I know is that the larger one is always the female. That way makes it hard to ID, as size is relevant. eg, if you have a pair of them for a few years, the female will get quiet large, but the male will still be pretty big too when you compare him to a juvenile.
As far as I know, juveniles can become male or female, males can become females, but females can't turn back into males.
With that in mind, you can see the problem with 2 females. 2 males is ok, as they will fight a bit to decide who gets to be the girl, but sort it out eventually.
So thats why a juvenile is best, it won't be seen as a threat to the female, so she will be more likely to accept it.