I think friendships just sort of happen. One day you just know you care about the person you're with, or are interested in what happens to them in their lives, or you simply have a lot in common and can relate, and have fun together.
When do you know? What defines it?
I liken it to the question people often ask about love. "When do you know when you are in love with someone?"
My response is always, "When you can't remember when you weren't in love with them."
Maybe its not quite that simple. Then again, it can't be that complicated.