I am a negative person, so I complain about bitchiness. ;-)
This is the most obvious of observations - though I suspect not so obvious to some. Have you ever noticed how when people complain about other people's behavior, they always focus on that one thing that they themselves struggle with? Selfish people always call out others as self-centered, people who don't have any follow-through complain about flakes, people who are kind of simple complain about stupidity, people who are a bit stuck in their personal progress complain about immaturity, etc., etc.
Keeping this in mind helps you become an excellent judge of character. When you are just getting to know someone, notice what he or she complains about - and then decide if you want to deal with that characteristic.
In case you're wondering, why yes, I am talking about you. Don't think twice, it's all right.
Update: Uhm, did I strike a nerve much? Plz to no one take this personally. I said I was talking about "you" because I'm not leaving myself out either. I meant "you" in the most universal and non-specific of ways.