I hate to say the same thing as XCDXER0, but honestly you really will know. At some point it will just seem to be the natural thing to do, and as long as you're prepared for the answer (whatever it may be), you'll be fine.

Also, I wouldn't worry about presentation all that much either. If this girl really likes you, it won't necessarily matter how or where it's said, she'll just be happy that you did. Stop sweating the small things so much and just go into it with all that you are.

And remember, we're all rooting for you! ^_^