Probably the forehead; it's a very strong indicator, but it really depends on brow bossing, the slope gradient, width, vertical height, hairline, the nasal radix when viewed from the profile, position of the brows, and the eyes. So there are a lot of factors involved in gender recognition (not just the forehead, but the whole face, but from what I've read the forehead is the most significant indicator), and in most cases it only takes nano seconds to decide male or female when we see someone, and if a decision was not 100% then we look for other cues and features to make a determination.