The older I get, the further away from college, and the longer I've been married, the more I realize that other relationships (social and professional) are an awful lot like dating.
I remember one test very clearly from my dating days: how someone treated waiters/tresses, grocery store clerks, etc, was very important to me. Of course a guy will try to impress his date, but is he as courteous to everyone else-- especially those people in service positions who are often ignored? How does he treat friends and family? Does he still show respect to people he doesn't enjoy?
I only put this together a week or so ago, and as you might imagine, it wasn't a fun lesson to learn. With friends, this awareness isn't necessarily the weeding-out process it is with dates, but it is a helpful perspective. If someone has a short fuse with others, she'll probably lose her temper with you at some point. If an acquaintance gossips, it's best to assume you'll eventually be the topic. We keep loving each other, but forewarned is forearmed, and assuming that we'll be different from all the others almost always ends in heartbreak.
And along the same lines... it's not a bad idea to evaluate ourselves the same way. Even when we're not called out on it, our friends put up with a lot.