I'm all for the "Are you single" line... as always, your energy, intention, eye contact, tonality and way that you ask the question is the most important thing.
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I like the idea of getting quickly to the point, and getting a straight yes or no.
I wouldn't look at things so black and white when it comes to seduction.
The reason to ask if she's single is to set the frame "I'm interested in you sexually, how are you feeling about me right now? Is there anything that's going to come between me seducing you?" and you will always get some sort of reaction from her.
The way that she reacts and the answer she gives will also give you information about what her situation is and also how she feels about you making a small advance towards her in that moment. If you can use your social and emotional intelligence well to her response, then you can then use that information to continue with the rest of your seduction in a calibrated way.
An example - "So are you single right now?"... Her response (Seems like she gets a little frustrated) "Well my love life is a little complicated at the moment"... BINGO... Now you know a little more about what you're working with.
Another example but as an assumption - "So I'm taking it you're single right now"... Her response "What gives you that idea?" You "Well you're sat down with a charming guy that you just met in the club and you seem to be pretty good with the flirting"... Continue from there...