How do you know when a girl is interested?

If you have this friend that you talk to good bit.. but not in person because you don't hang with the same group.. and you stated talking to her one day and the next you find out she likes someone.. so being curious you ask and she said shes not going to tell you.. in a seeminlgy suspicious way.. and its coming off as she always is flirting with you.. but your not really sure, but everyone she hangs out with and you hangs out with, they all know eachother and everyone is pretty close.. how do you find out if your the one she likes.. and if you can't find out.. should you ask her to go out on a date.. even though everyone knows eachother and are in close corridors