How do i know when a girl is interested in me?

ok well.... there is this girl i have been talking to for like 3 days. everytime we see each other we talk. she is a very sweet and friendly girl. and after school she either comes to my car or she invites me into her car. We sit there and talk about random things and we listen to our iPods. she likes it how i make her laff when i sing the playing song, but i do that all the time and i don't kno if she is coming onto me or just being nice. we have the same taste in music and perspectives. i have been getting mixed feelings when she is around. i dont kno if she is just being a good friend or if she is interested in me. i already took her out to eat ice cream as a kind of a spur of the moment kind of thing. she has also recently gotten out of a relationship that didn't end well in a text message break up. I just need to kno if she is interested in me and if she is, when is the best time to ask her to be my girlfriend?