My boyfriend has owned mallards through the years and they are pretty easy to sex at about 6 months old. Mallard males have green heads and you can see the green coming in around 6 months or sometimes even before. Also the females are MUCH louder than the males. The females have more of a quack and the males are a lot quieter and more like a low buzzing sound. You never said what kind of duck you have. That would help a lot.