Simple! At their age right now, the females will be quacking and the males will still be peeping!!! I have never done vent sexing but I hear it can be quite tricky. They are too old to vent sex, from what I understand. It is for day-old to few-day-old chicks/ducklings. You probably cannot get a result from vent sexing at this point and it can be dangerous for them now. So "quack" sex them! Do you have any quackers yet? The females should quack at about 3 weeks and it starts off as a grunting sound. The males cannot make this sound yet and won't quack until about two months of age and will have a hollow, whispery peep for quite some time until then. Good luck with your new duckies!!