Classically the females are larger, plumper with more arched backs and the main difference described is the front toe pads. Compared to the back ones males tend to have much larger heart shaped front toe pads whereas females have a flat topped toe pad more similar to the size of the back toe pads.

This isnt hard and fast tho - when I had a breeding 2.1 trio of these darts a few years back one of the males would have been female by the toe pad rule but was only sexed for definite by observing him calling.