[Symmetry]

The line is going to vary depending on the personality of the person you're trying to flirt with and how attractive they find you. That's just the way it is. This is why you flirt in a civilized manner first, and look for signs that they are or are not interested. And if you're paying attention you ought to notice if you make them uncomfortable, and apologize.

[colmvp]

"And if you're paying attention you ought to notice if you make them uncomfortable, and apologize."

I don't think you need to apologize. Their uncomfortable demeanor could be brought on by lack of attraction rather than offensive behavior (assault/harassment), in which case their negative reaction will be a clear sign to get out.

[mindcrime]

This is why you flirt in a civilized manner first, and look for signs that they are or are not interested.

Yeah, it's easy enough to simply be polite, throw out some very casual flirting, and the see if the other person responds in kind. If they don't, don't try to escalate the encounter. Somehow this seems to escape an awful lot of people for some reason...

[Symmetry]

Yup. Geeks (male and female) can be dense about flirting at times, though. I'd like to think that I'm more perceptive nowadays, but when I was a sophomore in college I was asked out and taken on a date without realizing it, for instance. So be aware that they might just be really enjoying the conversation and not realizing that you're trying to flirt. Which doesn't even mean they wouldn't be happy to flirt back if they realized how things were! So again, just always be willing to revise your assumptions about the situation, and always be polite and be ready to back out if you misread things.