How do Teachers Become Experts?

I'm taking a course on expertise, and how people develop and demonstrate expertise in different fields. What are the different ways expertise is exhibited? How do you know an expert? How do you become an expert?

So I pose those questions about teaching:

How does one know an expert teacher?
How does one become an expert teacher?

I have some thoughts, but before I muddy the waters with my bias, I'm interested to hear what others think. BTW, you will not be graded on this....