I think people always go about this the wrong way. The simplest, most straightforward, and best way to find this out is to ask someone on a date. You don't need to make it all complicated with awkward conversations that are uncomfortable and sometimes creepy if the girl doesn't feel the same way.

Just ask her to go out somewhere with you one night. If you really want to, include the word "date" when you ask, but usually most girls are smart enough to pick up on the fact that you're asking them on a date and not just a friendly venture. If you ask her out and include the word date, you don't have to go through all of that weird conversation stuff...she'll either say no and you have your answer right there, she'll say that you two can go but only as friends, or she'll accept a date.

You seem to know her well (hopefully), so what makes you think she likes you? And what kind of flirting do you two do?

Ok I AM a girl
And what u planned sounds more like an interrogation I agree with artzy. Just catch her alone and be all smiley and happy, but still call. Make sure you ooze (fake) confidence, ie even if you have to pretend. And ask her, "hey jess, how's it going?" "I was wondering if you would like to go out with me?"
Bang done. Not that awkward

Patrick just said it very well. Even the way you started in suggests the wrong direction. You should NOT be confronting her, but rather changing your own pattern to try to get a response out of her. Keep in mind that the signals you are giving might not be any more clear than the ones she's giving to you, so the only thing that you can do is change them. If you make it very obvious that you are interested in her, she might start showing the same feelings back.

... well my 2 cents would be that if you are already seeing her tomorrow night and it has been three months since you have been doing whatever it is between you, that asking her out on a date isnt really necessary unless you feel as if you can wait an extra week or however long it is until that time. if your ultimate plan is to do it tomorrow, then i wouldnt go with the blunt route because its not a blunt topic per say. a good way to approach it i guess would be to tell her how you feel about her and how the past few months have been for her and ask her where she is at with you. it always sucks leaving the ball in someone elses court, but at some point especially three months into something you need to know what is going on. just make sure you guys had a good night before asking her, if the night doesnt go extremely smooth then i wouldnt go ahead with your plan, only if you feel everything is going good

I am not the best with these sorts of things but i have this friend who i really got talking to a lot which then ended up into flirting, she'd try to sit nearby me all the time etc.

I was beginning to really like her (i still do i guess), but i'm not one to go dating or whatever, i thought she liked me back.... Perhaps she does, she told me things that she has never told anyone else before (probably because we're emo so she can talk to me more about her feelings). I made promises to her. She was always very happy and bubbly at first, but now she just seems to be miserable, lifeless and way more quiet around me.

I then found out she had a boyfriend, so that is probably why she is like this. Is she feeling guilty? I think so.

In my situation i would probably just be honest with her and say "Hey, i know you've got a boyfriend and that you're probably happy with him, which is what matters most. But i'll be honest with you and say i really do like you but i would like to stay as a friend to you. I'm open though."

Being honest is probably best, and it shows confidence or courage, which is really good.

Don't even fuck around asking her what you are, just make things flow how you want them to.

wait... also remember:

Nice teeth ^

Different people have different methods as to how you would aproach someone.