Pay attention to body language over emotions or verbal reactions. Legs crossed toward you, a good sign. Arms crossed across her midsection, a bad sign. They may be slighly guarded, but with good reason when approached by a stranger. Most women may feel insecure, let alone vulnerable when approached. Men typically don't have that strong of a response to a seemingly harmless exchange.
Women learn to play hard-to-get either by mothers

or by experience. Since guys are 90% of the time the one in pursuit a woman may get approached 1,004,892 times.. which can get a little tiresome. Also, its a guage on if a particular man is interested. If I semi-ignore you but you still seem confident enough to continue on with a conversation regardless, my interest is growing. Body language may change and the exchange may progress...
If nothing else, its a game. Men are the "hunters" while women are the "gatherers." You approach and we collect
