It is important to find someone who has not only been trained to sing, but also well-trained to teach singing. Finding a bona fide voice teacher can be tricky. More often than not a graduate of a university voice program - even one with advanced degrees - has become well-versed in their own vocal issues, but has had little to no training in fixing others' issues unless they were lucky enough to have attended a place which offers such coursework. However, rare is the degree program which offers anything more than a nominal class on vocal anatomy for voice majors. For those seeking voice lessons, this information is critical to know because most who graduate with a degree in voice will become voice teachers, but may not have been trained to teach it. It is a huge problem in our industry and a huge problem for the student as the effects of working with an untrained teacher can be hard to correct. Once ingrained in muscle memory, these bad habits can continue to haunt a voice throughout a lifetime. In the worst case they can lead to damage. You may have to ask very pointed question's about a prospective teacher's training background and the extent to which they have studied both voice science and pedagogy to get full answers.
Below is a great article written about choosing a voice teacher. Although it was written in 1997, the issues are the same today...
"More is demanded of the professional singer today than ever before. Not only must you have the voice but also you must have the brains. Not only must you have a good musical instrument, but you must use it for the expression of the thoughts and emotions that define and motivate humanity.
You cannot learn to sing from a book or a record. It takes personal interaction. You need a trained ear listening to you, encouraging you when you make progress and alerting you to bad habits that inhibit your progress.
It is also important to find a teacher who really understands the basic vocal mechanisms, and can effectively educate the student. My gripe with so many of the teachers is that they just sit and play scales and don't really explain how to get the most benefit from each scale. Today's teachers vocalize, they exercise, but they don't educate their students.
A teacher must be honest, and must be able to demonstrate the technique, and be able to show the control of their own voice. Never study with a teacher who is chronically hoarse. The teaching and the technique should be very natural. While proper technique may be difficult to learn, it should not make you feel physically uncomfortable. If a technique takes a great deal of physical effort to demonstrate, then it cannot be correct. This is one area where the old saying, "Those that can't do...teach," does not apply. If your teacher does not have a beautiful vocal instrument, beware. Even though the goal of study is not to imitate the teacher, a voice teacher should be able to demonstrate the validity of the methods he or she is teaching.
The job of a voice teacher is to teach you to control, and thereby free the voice to develop to its full potential. A teacher should not impose a style for you, for style and flair come from within. When you are vocally free to express you own feelings, your unique style will develop naturally.
Why do so many reputable teachers hold so many different opinions, whether they teach privately or in music schools? The main reason is that there is no single authority whose judgment on singing is universally accepted. Unlike any other branch of study, the teaching of singing can be practiced in any country without a certificate of proficiency. An untrained teacher can set up practice in any country without a certificate of proficiency. An untrained teacher can set up practice, begin to publicize themselves, and develop a large group of unlucky students. Even a bad teacher can set up a practice, begin to publicize themselves, and develop a large group of unlucky students. Even a bad teacher might occasionally find a student with an exceptional voice, which, with careful study, might have developed into an outstanding instrument. But by improper training, the voice may be damaged or irreparably harmed. Such self-appointed authorities probably never passed an exam and never had to explain what right they had to teach.
People come to teaching by different roads. Some teachers have planned to teach from the outset and use their good and bad experiences from their years becoming or trying to become singers to help others. Some start teaching after more or less successful careers. Some take up teaching because they never realized their own ambitions. Some teachers try to build up a large clientele by developing a stable of associates, that is, student teachers who are still studying themselves. These "associates," at best, parrot the method of the master teacher. At worst, they damage the unwary students they come in contact with, for they lack the experience to oversee the building of a voice.
Singers must examine their motives very carefully before deciding to pursue a career as a teacher. The general attitude, a patient temperament, is very important. The danger with some active singers who teach is that they sometimes compete with their students. The teacher has to feel love for the student, like a responsible parent, not like a baby sitter who dislikes children and only takes the job for the money.
This brings us to the financial motive. A reputable teacher should have asked themselves this questions: "Am I in this profession for its own sake, or only for the income I can derive from it?" The best motive for teaching is the love of voice and the joy of developing a voice to its fullest potential.
Apart from the ability to teach, it is critical to hear objectively as well as subjectively as singers normally do, to be able to diagnose what is heard. The skills are to recognize how a student is producing their sound, identify the things that inhibit the sound, and apply the correct remedy. In other words, the diagnosis is like that of a doctor's, who looks at the symptoms for a clue to the cause of the trouble.
Many good singers are not able to teach because they lack this ability, while some have a dual gift for singing and teaching. Coaches who are often unable to sing well themselves often lack the knack for teaching. Dedicated professional teachers are interested in teaching even during a period of successful activity in their own careers. If they possess the talent of listening critically, they may continue to learn much and apply it to their own careers. If they possess the talent of listening critically, they may continue to learn much and apply it to their own singing. Good teachers, too, can learn from teaching.
An objective way to judge a teacher is to hear his or her students at certain intervals. Any teacher can, by luck, produce one of two good voices, but the reputable teacher gives something to all students that makes them progress at their own optimum pace. "High notes created in the shortest time... technically faulty singing will be corrected... develop great vocal power...etc." An honest teacher could never promise so much. Students who hear these outlandish claims should be on guard. Such dishonest teachers will never admit their inability to keep their promises. Instead, my fault of failure is laid on the student's head. "I'm sorry to have to tell you this, but you have a physical defect that will never allow you to become a singer." Don't believe it. Unless a throat specialist tells you that you have a serious throat problem, you can sing!
Before I paint the perfect voice teacher, I would like to stress that teaching is hard work, a commitment which takes years. A teacher gives the advice gained from years of study and hard work, and earns every penny he or she gets. You don't get something for nothing in this world, and if you are benefiting from your voice lessons, then you should be glad to pay for them. Nothing worth having in this world is easy or cheap. And a career as a singer is a thrilling experience that only one person in a thousand may be able to have.
Don't measure your teacher by the dollar, however. The cheapest voice teacher is not necessarily the best bargain. Likewise, a voice cannot be bought by the pound, so that even the most expensive teacher may not be the one for you. The expression, "Penny wise and pound foolish," really applies when choosing a voice teacher. Find the one for your needs and stick with them!"
— excerpted from Breaking Through: From Rock to Opera, the Basic Technique of Voice by Gloria Bennett