Hi, going into our 6 year of raising chukars. What a beautiful bird...we find that sexing them is very difficult at a young age. However, at 4 to 5 months, it is clear by the shape of their heads and body size. Males are larger with a more square head. at 8 months or so, you can begin to see spur knobs...but this is not an accurate way to determine males or females as females sometimes have them too. In adult chukars, you can get about a 95% accuracy by looking at their genital part.
Turn the bird upside down, press with your thumb and index fingers at the genital area. If you see a point, it is a male. If you see an oval or circle, it is female. You can only use this method in older birds though...