If you are talking about adults. All of the breeds except muscovy will have a drake curl. Most, but not all, drakes will also have different and more flashy plumage. This depends on age also about 4-5 months for adult feathers. Some breeds you can tell at a younger age, 5-8 weeks. Pay close attention to how your runners sound. At 2-3 months of age, all ducks, except muscovy, will quack. So if they sound like your girls, they too are girls.
For muscovy, you will be able to tell the difference between males and females by size starting at about 2-3 weeks. After about 6 months, a muscovy drake will weigh around 9-10 lbs or more and a hen will only be about 5 lbs. If you are looking at younger ducklings, you need to ensure they are from the same hatch and then you can compare size.
For ducklings, the only way to tell is vent sexing. I am not experienced and have not yet attempted it as it can injure the duckling if done wrong.