lol i can never tell. I don't even realise until a friend tells me that he's hitting on me or osmething.
but there's one thing I have noticed: the guy always cracks jokes to make the girl laugh. (well.. in all my cases anyway..except I can never tell if the guy's doing that if he's interested or if he's just trying to be funny)
If you don't have the courage to bluntly ask him, ask your friend, to ask her friend, to ask her friend, to ask him