An expert can sex a duckling at day one, BUT I have read you have to be very careful with waterfowl. You can permanently damage them. Try duckgoose.com for info. that's Dave Holderread's site. He is the primo expert on ducks in this country.
When it gets a little older if it's a girl it will quack. You only have one...right? A good way to get them to quack is to take them away from the others by themselves and then it'll either quack or make it's raspy noise. If it's attached to you it might work for someone else to take it from you and walk off with it. They need to be a little older for that to work tho.
You have blue swedish right, my drake had tufeted like tail and he had a lower pitched quake, and if he is raised with chickens (Like mine) he might just get confused and start trying to mate with your chickens, I know that may sound...WEIRD! but!, its not at all my duck tryed to do it all the time and then he figured out that his duckie partner in crime is his mate