It's easy to tell an open-minded person from a closed-minded person because they act very differently. Here are some cues to tell you whether you or others are being closed-minded:

1. Closed-minded people don't want their ideas challenged. They are typically frustrated that they can't get the other person to agree with them instead of curious as to why the other person disagrees. They feel bad about getting something wrong and are more interested in being proven right than in asking questions and learning others' perspectives. Open-minded people are more curious about why there is disagreement. They are not angry when someone disagrees. They understand that there is always the possibility that they might be wrong and that it's worth the little bit of time it takes to consider the other person's views in order to be sure they aren't missing something or making a mistake.
2. Closed-minded people are more likely to make statements than ask questions. While believability entitles you to make statements in certain circumstances, truly open-minded people, even the most believable people I know, always ask a lot of questions. Nonbelievable people often tell me that their statements are actually implicit questions, though they're phrased as low-confidence statements. While that's sometimes true, in my experience it's more often not. Open-minded people genuinely believe they could be wrong; the questions that they ask are genuine. They also assess their relative believability to determine whether their primary role should be as a student, a teacher, or a peer.
3. Closed-minded people focus much more on being understood than on understanding others. When people disagree, they tend to be quicker to assume that they aren't being understood than to consider whether they're the ones who are not understanding the other person's perspective. Open-minded people always feel compelled to see things through others' eyes.
4. Closed-minded people say things like "I could be wrong . . . but here's my opinion." This is a classic cue I hear all the time. It's often a perfunctory gesture that allows people to hold their own opinion while convincing themselves that they are being open-minded. If your statement starts with "I could be wrong" or "I'm not believable," you should probably follow it with a question and not an assertion. Open-minded people know when to make statements and when to ask questions.
5. Closed-minded people block others from speaking. If it seems like someone isn't leaving space for the other person in a conversation, it's possible they are blocking. To get around blocking, enforce the "two-minute rule" I mentioned earlier. Open-minded people are always more interested in listening than in speaking; they encourage others to voice their views.
6. Closed-minded people have trouble holding two thoughts simultaneously in their minds. They allow their own view to crowd out those of others. Open-minded people can take in the thoughts of others without losing their ability to think well--they can hold two or more conflicting concepts in their mind and go back and forth between them to assess their relative merits.
7. Closed-minded people lack a deep sense of humility. Humility typically comes from an experience of crashing, which leads to an enlightened focus on knowing what one doesn't know. Open-minded people approach everything with a deep-seated fear that they may be wrong.

Once you can sort out open-minded from closed-minded people, you'll find that you want to surround yourself with open-minded ones. Doing so will not only make your decision making more effective but you'll also learn a tremendous amount. A few good decision makers working effectively together can significantly outperform a good decision maker working alone--and even the best decision maker can significantly improve his or her decision making with the help of other excellent decision makers.