Best friends do fall in love! You can tell when all of a sudden even though you know everything about each other your best friend asks you out or mentions that you mean a lot to him/her. It's difficult to explain but you will know.
Yes it can happen, as has. Me and my best friend ended up liking each other. You can't really tell, you just know. Trust me if he/she really likes you, you will just be able to tell if there really is some sort of relationship coming out of it.
Okay, you can't always tell. I know this because my best friend liked me through all of 6th and 7th grade before he finally got up the courage to ask me out. And then, well, we broke up, but hey, it happens. And my other best guy friend just recently asked me out, and I had no idea this time either. Maybe I'm just oblivious, but sometimes guys can hide their emotions really really well. If you like him, start commenting on stuff you like about him occasionally. If he seems to be doing the same, you can always make the first move. If he says no, say it's ok and you hope you can still be friends (trust me, if he's a good friend, he will stay friends) This situation is REALLY hard, I hope it works out for you!!!!!