Oh my, are you sure that's a duck? Looks more like a stuffed animal!
Once s/he's around 6 weeks old you'll either hear quacking or a more raspy frog like sound. Ducks quack sooner than drakes.
Ducks quack, drakes croak.
I can often tell by their feet coloring but that's not always accurate.
Males will get a curly feather on their tail.
Do you have any other ducklings so this little one can have some friends? Ducks are very social and enjoy having other ducks. If not, s/he is more likely to imprint on you rather than other ducklings. S/he will be very clingy.
I suggest getting a stuffee animal and a mirror for the brooder.