You can ask some people the direct question, What makes someone credible? With others, you may have to be somewhat indirect. Ask, Who do you think is really reliable and can be believed? Why? You may have to be even more indirect with some people. Ask, Who do think is a great leader? Why? The responses you get to these questions indicate each persons key credibility markers.
Once you know the important markers, assess yourself. How well do you measure up to the standards mentioned by each person? For example, assume your boss indicates credibility comes from an established track record of success. Review your track record. As another example, assume an important coworker feels that follow-through on tasks counts as an important credibility marker. Assess your level of follow-through on assignments. In some cases you will not have to do much to build your credibility because you already stand out in areas important to others. Your boss may already know you have a strong track record. With other people, you may have to reinforce your credibility. For example, you may need to demonstrate task follow-through with the key coworker.
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