Comfort

You know that you are really comfortable with a friend when you don't feel embarrassed to fart near them. Now, I say near them because I am not talking about lifting up your leg in front of them and letting one rip out the other end. I'm talking about a subtle release of gas in their vicinity to release built up pressure. They may or may not hear it however you don't really care if they do. You kind of just do it and realize after the fact but don't really mind.
That to me is a friend you are really comfortable with.