The great thing about ducks is that they're easy to sex by  two secondary sex traits - voice and plumage.  You just need the time and space to let them grow up alittle. 

By 6 to 8 weeks of age, voice sexing is possible.  Only the females/hens are capable of making the harsh quacking noise that ducks are known for.  The males/drakes can only make a much softer hoarse "wongh" sound.   To me, the drakes sound like someone is trying to strangle them.  If you have some of both sexes, the difference is easy to hear.  If you only have one or the other, it might be a little less obvious since you have nothing to compare to.  If your ducks are way out in your backyard and you can easily hear them from inside the house, they're probably females.  On the other hand, if you're standing right next to your ducks and have the impression they are whispering even though they are excited by your presence, they are probably males. 

Ducks are born with a very soft fuzzy down instead of feathers.  By 6 to 8 weeks of age they have pretty well aquired their juvenile set of true feathers.  Almost immediately, they begin to molt and will have aquired their true adult plumage by the time they are 4 to 5 months old.  The adult plumage for  drakes includes one or more curly feathers on the top of their tails.  It depends on the breed and on the individual how tightly curled those feathers will be.  

The females do NOT get any curly feathers on their tales.