I have 4 female ducks, and 2 male ducks. The 4 females are layiing eggs, and i want to incubate and hatch them, my problem is, i don't know which ones are fertile. Does anyone know how you can tell if they are fertile? Please help!!!

Incubate them.

The odds are that with this ratio they are all fertile. It would be surprising if they are not. The only way to tell if they are in fact fertile is to being incubating them. After a few days, you can candle them to see if there are blood vessels beginning to show. After a week the clear eggs are infertile, and the fertile ones will show signs of life inside. Hope this helps.