Generally, I would class anyone who's not sat in the corner on their own nursing their pint as confident.
Also, if someone come up to me asked to buy me a drink, compliments me or whatever. Ill thank them, accept and chat for a bit, unless their completley arrogant. I mean if they've risk to come over im not going to tell them to go away and make look like a loser in front of their mates. And if I do deline I do it nicley.
I can usually tell what a guys after and adapt what i do/say accordingly, so not to give the wrong impression and look like a whore. But no matter what their intentions im always flattered, and usually repect the people who have the courage to go up to a complete stranger. I couldnt do it without being completley intoxicatied.