I have Rouen ducks. The males go through a color change as they sexual mature. Other than laying an egg, there's not a tell I use for females. If you are seaking full grown for meat, you just weigh them and compare to the market standards for the breed. Sometimes we are dealing with crossbreeds of course, in which case your looking at a window of 8 to 12 weeks of age.How can you tell if a male or female duck has matured?