How do I know if a girl is lesbian/bi and if she's into me?

There's a girl I known for years but now she's 15 and I'm 14 and sometimes we will be talking about something and all of a sudden, we will be staring into each others' eyes for like a minute. does this mean she's into me?

I don't really think so. When I was 14 I've got a friend and we did those things too. She sometimes threw her arms around me and looked into my eyes. But, y'know, she still likes boys....