There are different ways to tell the sex of a duck. Most are easy and can be completed through simple observation with minimal interaction with the duck. These things vary slightly depending on the breed of duck, but there are similarities that remain. It is easiest to sex a duck starting at around 2 months of age, where the process can be done through listening and observation.
Look at the feather colours, which are one of the most reliable means for telling the duck's sex. This method is most reliable when the ducks are full grown because ducklings' feathers change colours. Males will have more vibrant feathers. Identifying features of male mallards are green heads and a white ring around the neck. These colours and patterns are used to attract females. Female ducks are normally characterised as having a more neutral colour to their feathers--normally brown.
Listen to their quack because they might be telling whether they are male or female. By listing to the pitch and intensity of a duck's quack, you might be able to tell the difference between a male and female duck. Male ducks have a softer quack than females, but it is cruder sounding. It has been noted to resemble the sound of a rooster. Female ducks are much louder than the softer male quack.
Observe the size of the duck. Raising ducks in a pen that has both sexes would be the easiest way to tell the difference between the males and the females because as with most species of duck, males are larger than females.
Check out the tail feathers. At about 2 months old, male ducks begin to show a long tail feather that curls around. It is significantly larger than the rest of the feathers. This tale feather will remain there, without change, through the moulting process, and it can be used as an accurate identifier for the duck's sex.
Vent the duck, which is direct inspection of the genitals. The vent is the opening of the cloaca. This is the single opening below the abdomen of the duck in which fecal matter and urine are dropped and the reproductive system ends. This is a dangerous process and should be avoided whenever possible. If venting cannot be avoided, look to an expert for assistance. The process is sometimes used by farmers if they need to sex the ducks in the early stages of life. Venting done at an early age increases ducklings' mortality rate. After turning the duck over and opening the vent, there will be one of two things. The females will have a cone-shaped organ that is easily visible; the males will have a pointed organ that might not appear for a couple of seconds after the vent is opened.
Steps 1 through 4 of sex identification are best completed after the second month of life. As ducklings, they are almost impossible to sex without venting.
Do not try to sex a duck through venting if you have not done it before. The process can be harmful to the duck.