Easiest method: Via IM or e-mail, begin with "I have ... Well ... A rather akward question to ask..."
Wait for a response of "Ok," "?," or the like.
Then simply ask if she feels the same about you as you do for her.
If she replies positively, then enjoy the cyber-smex.
If she replies negatively but sympathetically, a simple "Oh, ok. So how 'bout them -insert generic masculine thingy here- ?" will make things normal.
If things turn uncomfortably negative though... Block her on every method of communication she might employ. Since it's long-distance, you never have to fret about embarrisingly seeing her again.
Though, if you want to learn the dangers of long-distance relationships, I can easily relate back to you my personal experiences with the boy who I discovered was a bondage furry who liked feet and being peed on, and the 23 year-old I actually MET UP WITH at Anime Boston '09 after 8 months online only to discover he was a creep in real-life that enjoyed fingering my Indian bread in the food court while slowly attempting to grab my penile gland with his free hand. Not to mention he openly admitted to wanting to be dominated by a Pikachu. Both seemed like entirely normal people when I first clicked on their contact info online.
I've since discovered that close-distance relationships are much more fulfilling and smexy.