If you want to be sure or just learn how to vent sex, it's really not that hard. I watched a video by Metzers. Then when I mistakenly got a 5th duckling with my delivery a couple of weeks ago, I could tell it was female.
They look like Rouen crosses to me...mainly because of the eye stripe, but some of the coloring looks familiar, too. I have 2 female Rouens that are 23 weeks old. Definitely beautiful ducks you have there.