LOL, I was wondering about your question, but I'd never leave an amiga in a lurch. So, around 5 or 6 months, sometimes sooner, male ducks develop a curly feather or two on their tails. Its usually right where the back meets the upswing of the tail feathers. Females don't ever have a curly feather. They are smooth.
You can also tell ducks apart a bit sooner from their vocal expressions. Pick them up at 3 or 4 monthes of age 1 at a time, and let them vocalize for you. Girl ducks have very loud clear QUACK QUACK QUACK sounds. Boys have a more muted/ muffled/ off sounds noise. It is NOT a good clear QUACK. I mean, it really sounds like the word "quack" when a girl says it, but boys are just like 1/2 power, and its not really a quack, more of a wonk or something. When you hear one quack vs one boy wonky noise, you'll forever hear the difference.
I'm really sorry your folks didn't get you the shed. Are there enough scraps about for you to make a frame and make your own? Maybe fully covered cages with front doors would be good. Scrap aluminum, wood, old plywood, backs off old pianos, whatever you can come up with to block wind on sides and have at an angle for roof on top would provide shelter. 1/2" stryofoam insulation is really cheap at hardware stores, and is easy to push into place after cutting inside your box to help insulate for them.