creaturelife is right, unlike chickens, you can vent sex ducks...as boys will have little "things" and girls won't. The trick is that it does take practice and there is only a 2-week window after they hatch in which you can do it. Basically, you turn the duckling upside-down and place your thumb and forefinger on either side of the vent and squeeze gently...a little fluid will come out, that's okay but you should also either see a little worm-like "thing" or the absence of one.....there may be another more in-depth article on here about vent-sexing ducks, OR Metzer farms has a video of it on their website.
Otherwise, you can wait until about 3 months when the ducks will start to reach maturity...drakes will have little curly-Q tail feathers, and they have very quiet raspy sounding quacks. Females, on the other hand will let loose really loud, duck-call like quacks.....and start laying eggs....there are videos of male and female quacks on youtube if you want to hear them. Good luck!