When are girls flirting with me?

Your look very approachable and nonthreatening - so they feel comfortable approaching you. You may look like the 'guy-next-door' so they feel okay approaching you. If you interested in the girl, talk about, joke around, move your body closer to hers. Make her comfortable without coming on too strong.

Now, if they are talking to you in a bar situation. Most likely they just want a drink. I don't do that. But, a lot of other girls do.

Just use your judgement in that case.

They like you. Ask one of them out or get to know one of them then ask one out. Flirt back if you like them or one.

I think if a girl is hitting you it means either one of two things. She is really angry/she really doesn't like you. Or if not in an angry manner than she is simply breaking the physical barrier meaning she likes you. If you like them or her then you should ask her out.