How can I better identify dangerous or immoral people in my life?

I don't like to be morally judgmental about personality and other optional differences. In fact, I like being friends with a variety of kinds of people: that expands my own horizon. Yet I've been prey to some really awful people in my life. Looking back, I'd have to say that I ignored some "red flags" -- dismissing them as mere optional matters, as opposed to signs of moral failures. How can I better differentiate "interesting" and "quirky" from "crazy" and "dangerous" in people that I know?