The feathers of green peafowl are very similar between the rooster and hen especially for young animals; it is very difficult to determine the sex. The best sign are the feathers on the front side of the neck. When they are approximately six months old the hens (right side) are getting feathers with a light banding and sprinkles. The young cocks (left side) will never has this light marks in.
If you want to know it earlier you have to make the sexing with a DNA testing.