How old are they? Those two aren't quite old enough to sex from plumage (drake feather), the best way to sex them is by their voice. Females are very vocal and make the iconic QUACK associated with ducks. Males are quieter and have lower, raspy voices.
They are about 6 months now. They do seem to have a quiet quack, quite husky. I have added another pic. All 4 ducks seem to pretty much the same. I have younger ducklings and have no idea what sex they are but I think they are far too you to tell yet.
what breed are they? they almost look a bit Muscovy and pekin?? anyways, mallard derived as said drakes have a raspy, hoarse voice whereas females have a full on quack. Muscovy however don't follow this, as they are not mallard derived, so it's all size, and the drakes only hiss, females trill, coo, hiss and honk(quack)
Mallard derived drakes also get what is known as a 'drake curl' but usually voice is the first way to know if you have a daisy or a Donald.