If she has any attraction at all whether it be natural or strong (at the gut) level, her actions will tell. In other words her body language.
If you get no signs of interest from her, you're just wasting your time. It is important to only go after (or better yet, let them come after you) girls that are interested.
You don't even have to weed them out if you're a guy with standards. They'll show you interest.
Case in Point:
I was at a house party and we were talking about IQ of all things. Anyway .. this HB 9 started taking an interest in what i was saying, next thing you know, she sits right beside me and starts giving me signals. She said she needed someone intelligent to talk with etc.. and was sick of 'stupid' guys (or something to this effect) in general. Imagine that. Though it seems rare, she was looking for intelligence or SEMI (Search for Energetic Males with Intelligence) lol. She had a boyfriend, but was saying how she was going to break up etc.
Anyway, the best way is to be yourself i think. Act kewl and calm like nothing can shake your confidence and will. Be casual, funny, and energized.