Not saying this goes for all women, but it has held true for the women I've met. They automatically assume that every guy they meet finds them attractive. They expect to be hit on and ogled over. If you want thier attention then be indifferent.
If a woman finds you attractive she will do something to get your attention indirectly. A flip of her hair, a tiny smile at you, moving around to stay in your line of sight, something. The odds of her making the first move are slim, but she will give ample chances to do so. She will give you a window. If you don't jump through it then it's over before it begins. Show some self confidence, take a breath and give it a shot.
After your initial come-on (not giving away my line) start paying attention and watch her reactions. The reactions will be subtle but simple. Things like leaning in closer, crossing her legs with to top leg in your direction, slow blinking, or maybe touching herself (not like that... nasty minds) touching her face or her neck or some other visible part of her skin, etc... Whatever it is, it will be small.
Like I said in the begining, this doesn't apply to all women. Just keep your eyes open and try to notice the small stuff. And girls try to remember... the male eyes are generally dull and likes curves and shiny things lol