It is a lot easier if you have two to compare but if yo only have one you just have to listen closely. At about 8 weeks old my females voice started to change. It changed from the duckling noises that they make to a more quack sound. The male kept that duckling squeek. The "quack " that you think of with a duck only comes from females. The males make a hissing raspy noise. If you can't tell by the voice then at about 12 weeks males get a drake feather. The drake feather is at the end of the tail and it is one or two feathers that curl over. Only drakes get that. So at the ealiest
maybe 6 weeks if the female is going to start quacking soon and latest around 12 weeks old. Then you will deffiantly hear the quack or raspy noise or see a drake feather.
Ebony my male (see the curled feather at the end of his tail)
Cocoa my female (notice her straight feathers at the end of her tail, plus she has an "egg belly" in this pic, males don't get one like that)
Here is the link to "sexing ducklings" that is here on the Sticky Posts in the duck section of BYC.
https://www.backyardchickens.com/forum/viewtopic.php?id=181147