A smile from a woman is not just a good sign, it's the #1 indication of interest, especially if you are seeing someone for the first time. It's called a buying sign. If she holds eye contact and smiles you need to get over there. It won't matter what you say. She already likes you by that point and body language is 75% of communication.
However sometimes it might just be a "I think you're cute" smile but they aren't available. In that case just let it brighten your day, and know that other women of that caliber will think the exact same things about you that she did, especially at that moment.
When a person likes what they see their pupils will get bigger so they can see more of it (they :gives you the eye"). They almost always smile in that case. If a person doesn't like you their her pupils get smaller to block you out (they "give you a dirty look"). They hardly ever smile in that case. Subconsciously, we are trained to perceive the difference.
For example, last week I was walking through a crowded outdoor concert and it was mostly old people but suddenly I turned to my left and saw a real beauty, only a foot away. She looked deep in my eyes and then I got a big smile, but it was just as my momentum carried me past her. I didn't process it until I was a few steps past and it would have been too weird to stop and go back (though if you are brave enough, do it).
I stopped for a drink nearby to give the situation a chance to develop. She glanced my way but she didn't send any more overtures. It was a dance which personally I can't stand because you can't hear and you feel obligated to ask them to dance (or they fear you will). But she was one of the prettiest girls I had seen in a long time, so I knew I was having a good night. So I went to another bar and got the phone numbers (and dates from) 2 other hot girls almost immediately. But God I wish I talked to that other girl.
A man's character is judged by the decisions he makes in a split second. Remember: sometimes you'll spend years thinking of missed opportunities, but you'll never regret having been shot down.