It's really too soon to tell. From 6 weeks old the males continue to grow but the females don't
The females get a sweet little gurgling voice that drakes do not. Once there are gurgles you know for certain which are female
I had Max who was thought to be male as he grew so rapidly as a rescued duckling, but he stopped growing at 6 weeks old and then it became obvious Max was Maxine when she started gurgling.
I have Daffy who I thought was female as he was small and gentle. He kept growing and growing and became a 14lb magnificent drake. He's nolonger gentle -- when he follows me round the backyard, he is fond of biting my shins and ankles.