I have always vent sexed my ducklings its actually very easy.
How to hold the duckling
Instructions
1. Select a warm place so the ducklings do not get a chill, or cold.
2. Hold the duckling in the palm of your left hand with its head downwards, and with the belly outwards. Take hold of the tail between the thumb and first finger of the right hand. With the second finger of the right hand, press in slightly inwards and upwards. This will expose the vent. Then with the thumb and first finger of the right hand, press the vent back and open it up. Do this very slowly and gently. This last movement may make the duckling empty the contents of its lower intestine. if this happens, gently wipe away the droppings with a rag and look closely at the vent.
3. If it is a female there will be a small cone shaped organ ending in the vent. However, if it is a male the vent opens more and a small pointed shape appears now and again. Sometimes it stays in view. This shape is small and only about 2 mm in length. Sometimes it will not appear for about 25 seconds after pressing and then may only show for a second or so.
4. If the ducklings are of the Khaki Campbell breed the female the bill is about 0.5 cm shorter than the male bill and is also "dished" or curved. The bill of the male is 0.5 cm longer and is flatter. Also the duck's heads are rounder and the drake's heads are flatter.
5. If an egg laying breed, you may wish to kill the males for meat when they are 1½ to 2 Kg in weight, and only keep one male for breeding.
If it is a male you will clearly see a penis like structure which can be black or pink in color. If it's pink it's much harder to see. If you don't want to vent sex you can wait until they are about 8 weeks old. Females have a LOUD sharp quack while males have a whispery raspy quack. Good luck!