How to know if she likes me too?

[Jason]

Okay so I'm in 12th grade and In High School. I've known this girl for a while because we're both in advanced classes so our friend group overlaps a little but I haven't really talked to her much. Around last year I got to know her more because a group chat with about 10-12 people was created and we were both in it so we got to know each other a little better. Then summer came- we didn't talk, and I didn't really have an interest in her leading up into senior year. Now that I'm in senior year I thought it would be a good time to overcome my shyness and talk to beautiful girls- and boy is she gorgeous! Long, dark hair, slim body, pretty face, the whole nine but she's also wonderful on the inside. I'm not the type of guy who just likes a girl off of sheer looks although I do appreciate a pretty girl. She's one of the most down to earth girls I know! Anyways, I inboxed her on facebook a couple of times over the past week or so and we've had some conversations via facebook to where I feel pretty comfortable with her. Our convo's have spanned from about 8pm - 3/4 am with various breaks in between. She also gives semi- in depth answers, not one word answers. Since we're off from school this week I haven't been able to see her in person and gauge her behavior around me to see if she might be interested in me - I'm okay looking, slightly overweight but the weight is well displaced, I've been told I have nice hair, and I'm tall; however I don't get much female attention. I'm just wondering what I can tell from the conversations we've had if maybe she's interested in me... just a little? What are some signs? Also, how do I know how many times I should start the convo before waiting for her to start one? She did sort of start one, I asked her to play a game with me and the next day she inboxed me saying " I can't believe you did that! " and a convo stemmed from it. Also, I have her number but am nervous about when/ how to call her. I don't know if I should ask outright or... I'm nervous. I'm not used to dealing with girls this all around amazing.