Now seriously the pic below is of swords but the same holds for Mollies Platties, Guppies etc. Just ignore the sword tail portion when looking at the other fish
Ignoring the swordtail on the male we can see the difference between the anal fins of the make and female. His is like a lil spike sticking backwards (he can actually turn and manipulate the angle when mating) where as hers is fanned out more like a normal fin. Female as in the picture is usually more robust vs the trim male.