Women's hair is a source of power and confidence to them -- why else would they get so devastated after a bad haircut? They tap into its power at key moments, subconsciously unleashing its seductive potential. If you see her twirling her finger through it or throwing it around, like in a shampoo commercial, then you have a potential flirt in your midst.
This goes for body language in general. Some women like exposing their necks, prepping their clothes, or placing their arms in front of them in a way that their biceps push their breasts together, augmenting their cleavage. Some magazines tell women to let their shoes dangle at their toes, displaying the curvature of their feet, which men, apparently, associate with their other curves. However, if she's crossing her arms, it means she's distancing herself. Be alert.