Now that I've read that she is a sophomore in high school, I think back to the days I was in your shoes. Ah yes. Girls are scared shitless in high school, a big social scene they get to strive and bloom in, they're meeting these new people, they're experiencing emotional attraction (and physical) for the first time..they are clueless. Don't be afraid to go up to her and break the ice, because thats the only way things will get accomplished, don't sit back and wait for her to say something. You have balls, the person with balls makes the initial verbal contact. Suck it up and call her, it's ok to ask her to do something over the phone...why do you think cell phones were invented..COMMUNICATION. USE IT. COMMUNICATE WITH THIS BROAD. Don't be so blunt and ask about what emotions she's feeling, trust me they get really uncomfortable when you get all up in their girlie girlie sense. Never say you understand either, whoosh, no don't say that, especially if she's crying about something.
Anyway
Just call her spur of the moment and be like hey you wanna do something, example: minigolf, the sold-out classic movie scene of a first date, though it won't technically be considered a date in your circumstance. A movie may ...how do I want to say it... spike her curiosity with what you have planned (if she already has a clue you're trying to test her reaction to the possibility of a relationship). Ok so go do something that's physical, like the minigolf example, go and play that and sort of become a slight bit flirty, I know it's hard, but your balls have to touch the ground some time in your teenage life. Point being, do something with her that expresses your greatest assets, if you're humorous, put on your best gig (don't be cheesy about it, and if you see she's already in a bad mood, make 1 joke and see if it works, if not...don't bother continuing because that'll make you look like a complete tard). It's all about communication, girls at that age are not about expressing emotions...especially about a new-found "guy thing" when she's feeling attracted. More times than not girls at that age are EXTREMELY shy, so it's up to you. She wants to go out and have fun, I know you two have been friends for too long already and bringing up this subject is awkward and sort of...out of place...but it doesn't always have to be verbal. You can make strategic gameplays to hint any girl into realizing you like her. You don't have to kiss her or anything, and certainly don't go grabbin that ass all so quickly, you need to figure out how she reacts to your mood and personality (be fun about it, just having an all-out good time will convince most girls).
Figuring out whether or not this long time friend has "those" types of feelings, well, thats difficult. In my personal experiences at that age, when you're so knowledgable about the girl and you two are comfortable to go out and do routine things like getting coffee...making a move like hugging her out of nowhere may pleasantly surprise her and she'll go with the flow (in which you non-verbally make a mental and emotion agreement that states you two are now comfortable at the next level). Like pulling her arm to get her closer to you, she may actually like it and not think it's rude or out-of-nature...it's all things you have to do in order to figure out your boundaries with her. I'm not saying go in for the big "moves", but you need to examine her mind a little bit. Just don't be afraid to call her...you'll be nervous going out with her, knowing that you like her and unsure if she likes you, though thats all apart of the fun!
Like I said, they are learning at that age at a VERY rapid pace, sometimes overwhelming to them, but its also when they have the time of their lives...and if you make it happen for them...they will quickly grow up, and it will benefit you as well....