Males have lots of red caruncles around their eyes and face. These are red wart like things. Females have them too, but too a much lesser degree. This picture shows a male and a female. Males also sound different from females.
Males sound like they have a sore throat.
The females have a much clearer honk or quack sound. Muscovy ducks make great layers and setters. The eggs take 35-37 days to hatch, and they breed from March until October or sometimes even November. One hen will often have more than one nest full of ducklings per year. You will notice that the hens will lay one-two eggs per day.