Talking is definitely the best policy (isn't it always?!). I know that's easier said than done, but being best friends, hopefully you'll be comfortable enough with each other to talk it through once you get started and work it out. If she doesn't feel the same, as long as you have a strong friendship, which I guess you do, you should be fine, even if it is awkward for a while.
Although I don't usually go for the straight-up methods, I think The Mudman's idea of asking her out on a date may work too; you could drop it playfully in, and if she doesn't like the idea, you can either go as friends, or say it was just a silly idea; play it casual. Perhaps. I still think talking though.
EDIT IN REPLY TO NEXT POST:
Cinema, in this situation, would be good I think. Usually it's not the best date setting, but seeing as you know each other already, you don't need to talk so much, and all the other places I can think of, you've probably already been as friends. Doing something 'cliche' could help it be a date and not just a normal hanging out. Good luck!