I don't know if you can vent sex at that age... I've only heard of vent sexing on ducklings, but maybe. You may want to ask this question in the duck section.
If it quacks it's a girl. If it rasps quietly it's a boy. Only girls can QUACK that loud noise. Later in maturity a boy will also get a curled up feather above his tail on mallard derived breeds, like Pekin.