You need to talk to her about it. That sounds scary, but you don't need to come barreling out of the gate with all of it. Try talking to her one day, and just kind of make it clear you're curious about it. Something like: "I don't want you to get too freaked out by this but I just wanted to ask, have you ever considered me as more than a friend?" DON'T tell her you're in love with her, tell her you're developing feelings and you want to know if that's something the two of you could consider.

If she says anything from "i dunno, maybe..." to "YES!", what you want to do is a trial date. Set the rules in advance - you will both treat this like a date. You will not be two best friends, you will be a guy picking up a girl he met for a date, and you're going to ask each other questions and get to know each other as if you don't already - I mean you don't have to totally pretend like you're strangers but you just want to separate it from your normal friend things, relearn the relationship because if this works out, it's what you're going to have to do. At the end of the night, if it goes well, you kiss her and leave. Whatever you do, don't try for more. You need to both take this very slow so that nothing happens before you might change your mind and it'll be too late to take back. After the date, you talk about it. Maybe you do one more. Maybe another. Do it one date at a time, don't fuss about exclusivity yet and defining what it is, you are a guy taking a pretty girl out for dinner, drinks, a movie, a show, whathaveyou, you HAVE to think of this as a new relationship, because it is, just reborn from an old one.

If she doesn't seem interested, you take a little space. It is never a good idea to be best buddies with the person you're getting over. That's not permanent, just tell her you would like a couple weeks apart to stop yourself from developing further feelings and you promise her it's not the end of the friendship, but rather the break so that you know the friendship can continue.

I'm really rooting for you, a lot of girls find the best friends in love thing so romantic. Hopefully she's one of the those girls too! Just make sure you know what you want going into this, if you take it slow you can still go back to being friends but nothing can ever be undone, and it will in some ways change the dynamic of the friendship, but being in love with her now already has.