Adults are easy to tell apart. 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.