Some girls will tell you otherwise but the girls I know say that they know within about the first thirty seconds of meeting a guy whether or not they want to date him. My closest friend actually has a third category, whether or not she wants to sleep with him. That's probably because if a guy is self assured, nice to be around and physically attractive you know it within the first thirty seconds.
Just treat her how you would want to be treated if you were a girl. Two guys, guy A is always friendly and nice to you, compliments you but seems to do it more because he wants you to like him rather than because he actually thinks so. As a matter of fact, everything guy A does seems to be because he wants you to like him. You're not very sure how much he likes himself, he doesn't seem to have much confidence in himself. He's still a nice person though, just romance with him is a bit of an awkward thought.
Guy B on the other hand is also friendly and nice to you, he's always telling jokes and he's really funny. You can have a down to earth conversation with him about pretty much anything, he makes you laugh and when he compliments you it makes you feel good because you know he means it. As a matter of fact, being around him just makes you feel good and you find yourself looking forward to his company. You get the feeling he feels the same but you're not sure, you realize you like him and wonder if he likes you too.
If you were a girl, could you really say you would date guy A? I'm a guy so girls feel free to step in and correct me but doesn't guy A sound a lot like the proverbial Nice Guy? He seems nice enough, there's nothing wrong with being his friend but being his girlfriend is another story. Personally, I would go out with the second guy but everyone is different. I had a conversation about something like this a long time ago and this girl completely reamed me out, saying if men date insecure women all the time that girls can date insecure men. None of this is set in stone, just my take on it.