Just read her non-verbals before you even open your mouth. The easiest way to do this is to make direct eye contact with her before you even talk to her. If she smiles at you, it's probably on. If she holds the eye contact with you for an extended period of time, it's most likely on. If she blushes, it's most likely on. If she does any combination of the above, it's on.
When I say "it's on," I mean that she isn't going to turn down a friendly interaction with you. She might still reject your romantic advances, although as soon as the above non-verbals get combined, the odds of your romantic advances getting rejected diminish greatly.
As a general rule, I think it is poor form to make your romantic intentions verbally explicit with women. They respond much better to non-verbal indications of romantic interest. Nothing says "I'm interested" better than direct eye contact, imo. You can be talking about the most boring, mundane things, but if the rhythm of your voice is seductive and your eye contact is direct, you are flirting with her. Add in some touching and you are no longer flirting with her, but rather, seducing her. Her non-verbals are going to be practically yelling at you to touch her if she's stuck around for >5 minutes of you looking her right in the eye, talking quietly/deeply, and sticking to light conversational material. Obviously start with friendly areas like touching her shoulder to accentuate a point (I start doing this right from the beginning) and start standing closer to her.
I personally have NO idea how to tease women (or people in general), which means my verbal flirting abilities are poor as hell because teasing is 99% of verbal flirting. While I am a master of the so-called 'neg', I feel like I simply make women very insecure and hurt their feelings by doing it. I don't like to do that, so I don't 'neg' girls anymore. I used to do it with girlfriends, and eventually, they'd begin to feel like everything they did wasn't enough.
It helps a lot to be a little bit taller than average, good looking, well dressed/well groomed, confident in the way you move/hold your body/speak, and not fat.