It's hard to tell sex until they're a bit older. Adult males will get a curly kink in the end of their tail. Also they will quack differently. The males have a quiet croaky quack, and the females are much louder and make a more typical quack sound.
I have runners, 2 ducks and a drake, and one of my ducks went broody so I just left her to it. Now we have 3 ducklings, 2 weeks old. They seem happy, I just make sure they have food and water. Their mother is very protective of them and hisses at me when I go near. I have had to separate them...