When I approach a girl, I make it pretty obvious I mean business. I mean, pleasure, not business, but yeah you get the point! Anyway! So if they're taken or otherwise not interested,
that is the time to tell me, so we can move on and have the minimum amount of mutual awkwardness. (Which is
slightly different from mutant awkwardness.)
But in general I will assume everyone is single until told otherwise. It's just a good attitude to have, can't be half-assing it when you try to get laid. If you're super careful, it's just a turnoff. Hell, they might even make up a boyfriend just to get you off their backs, I've actually
been the surrogate boyfriend for girls who got annoyed with a guy hitting on them, so I know it's not
beyond the fair female mind or anything like that. It doesn't just happen in the movies. And, I'm quite sure guys do the same thing too! Though I don't know if we're as sensitive as that...
And if they continue to flirt with me even though they are seeing someone else (or don't tell me at all), I don't feel all that responsible for any consequences it might have.
Also, I agree with Sian that people who are in relationships namedrop their boyfriends pretty much all the time. Basically, I just don't worry about it too much, I guess.
Jam it back in, in the dark.