There are a lot of people in this world claiming to be master teachers. Some of them deserve that title by virtue of their life accomplishments but some of them are more like “master artists” rather than master teachers. Just because they have had successful careers as singers, doesn’t mean that they are great teachers.
A master teacher is able to find that which a student needs in a kind, fair and direct manner. He or she is able to target that which is most necessary and address those issues swiftly, but not in a way that is rushed. He or she is confident but not over-confident, secure but not  infallible, and enthusiastic about teaching rather than bored or condescending.
There are all kinds of ways for a teacher to possess mastery but not too many of them are going to show up in someone young. Wisdom more or less implies age, as life experience is worth something. I have heard about younger teachers calling themselves all sorts of fancy things in order to establish their expertise in the world. Doesn’t really hold up to scrutiny.
A master teacher isn’t out to impress anyone. She is out to be helpful in a practical, simple manner. She is out to give assistance in a way that empowers the person she is teaching and illuminates something about their singing or their artistry. She isn’t interested in demonstrating how much she knows, she is interested in discovering how the student can come to know, in any way that works.
Sometimes master teachers are nearly invisible. They are not famous, they don’t have flashy websites, they don’t take out big ads in local papers, they don’t really promote themselves. If you didn’t look you wouldn’t even know they had something to offer. It’s not necessarily so that people who promote themselves in the above ways are fraudulent, but if all you buy is the window dressing, without actually looking for substance, you can be fooled. Be aware, seekers, you have to look for that which shines out from underneath the professional persona.