I notice the little things that tell me someone is a good friend. For example, when you fall out with a mutual friend and, while remaining firmly neutral, they say they were concerned that you'd turn against them too. If a catastrophic event in your life has led to you not eating or sleeping properly, they may be the only one who notices that something is wrong, and will ask if you're okay. They might notice that you're feeling lonely, even though you're out somewhere with others present, and come over to talk to you. If you tell them that someone is saying the worst things about you, they'll choose to see the best in you. When your fiancée comes for a visit, even though they've never met before, and your friend is not generally comfortable with people they don't know, they'll do their best to make her feel welcome and a part of everything that's going on.
I'm talking about someone specific, and I'm sure I could use many other examples. Friends rarely tell each other how much they appreciate these things, though. The strange thing is, I've only known her for a number of months, rather than years. However, she's one of the best friends that anyone could ever have. I just hope that I'm as good a friend to her. Now, if you'll excuse me, I have to get back to being a lot less serious, and pretending that I merely tolerate her existence.