How do you know if someone is your friend?
This one is at once hard and easy. To begin with, I think friendship is an act of faith. To have friends means to make ourselves vulnerable, to trust in someone else. I don't think you really know if someone's your friend until you have to trust them, and they come through when you need them. Conversely, you can go months without talking to someone and still know they'll be there for you...that's a friend.
We make friends in all kinds of ways. I have people I would consider friends, whom I trust, that I have barely met in real life, and others I haven't seen in years. With some people, friendship develops slowly, and in other cases, I begin to believe in friendship "at first sight"--when we have an instant connection with someone. Friendship really is a kind of love...it comes in degrees, it can grow over time or happen instantaneously, it asks us to put someone else above ourselves, at least on some level. We all express friendship in our own individual ways; each friendship has its own language.
I know someone is a friend when they keep the confidences I share, and when they trust me with their own. I know someone is a friend when they go out of their way (even a little bit) to think of me or do something for me, and I enjoy doing that for my friends. We know someone's a friend when we're happy for them to call on us for help, and comfortable calling on them in return.