He who knows not and knows not that he knows not is a fool, shun him.
He who knows not and knows he knows not is a student, teach him.
He who knows and knows not he knows is asleep, wake him.
He who knows and knows he knows is wise, follow him.
The above illustrates the point that one should be wise in choosing one's guides, whether a mentor or a coach. The person who is the most knowledgeable is not necessarily the best person. In seeking a coach, s/he should have demonstrated success in the area of expertise s/he is coaching but should also be able to assist you in transferring that expertise to you in a way that allows you to maintain your own uniqueness and integrity. A mentor needs to be someone who engenders trust in you and who can look to your welfare and development instead of focusing on sharing his/her experiences and expertise. Creating that climate of trust is the most important skill your mentor needs to have. So when thinking of someone as your mentor, it's important to engage in conversation and "listen" for how that conversation makes you feel. Does your prospective mentor make you feel at ease? Is that person open to hearing what you have to say without interruption and without preconceived notions of how you should do something?
In both a coach and a mentor, look for someone who is self-aware and knows his/her limits as well as their abilities and is focused on you and not on them.