Know Their Limitation
Some time ago, a study was done to see how people view themsleves as far as knowing things. They gave a bunch of folks a math test. The test went from easy arithmetic through calculus. After each person finished the test they were interviewed on how they felt they did. Some of the test takers were bragging on how easy the test was and that they aced it. Some of the subjects felt confident, but they had some lingering doubts about some of the math problems. They thought they did well, but were a bit nervous.
They then scored the tests. The people that bragged the most did the worse. The people that had cautious confidence did the best.
The point was, that many folks that brag and act like they know everything are actually hiding a lack of knowledge. People that are truly experts in a particular field are confident, but not obnoxious about their skills. I have found this to be the case in dealing with others. Generally, the more a person brags about stuff, the less I am impressed. I have more confidence in a Doctor that occasionally utters the words, "I am not sure," then any MD that doesn't feel they make mistakes.