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...
Girls will quack fairly early, while the little drakes just make a quiet raspy noise. Other than that the drake curl on the boys will be the most telling thing.