Mentors don't necessarily have to be omnipotent. I believe a mentor is picked by how much life wisdom they accumulate over the years. Anybody can be a mentor. It's a matter of being open-minded. This is an important skill a mentor must have when teaching students.
"When the student is ready, the mentor will appear."
The only measure is the sincerity of the student's need. When the need is legitimate, the student will suck up all pertinent information like a dry sponge. A good mentor is one that never runs dry. It's pretty obvious when a person you value as a teacher has no more to teach you.