First of all, you must be able to discern whether or not a teacher is primarily a voice technique teacher (one who shows you how to sing), or whether he/she is primarily a voice coach (one who shows you what to sing). Of the two types, the voice technique teacher is the most important, because without the technical ability to sing flexibly and clearly in all parts of your range, you are very limited to the material you can do.
For the initiated, a good voice technique teacher is hard to find. Many so-called voice teachers are just vocal “cheerleaders,” who bang away at a piano while you follow along. That is not teaching you how to sing, however. You just get a lot of practice following a piano, and memorizing the notes of a song.
Furthermore, when the teacher’s methodology consists mainly of using terms such as “give it more support,” “sing from your diaphragm” and “open your mouth,” you know you are in the wrong company. If you don’t feel your voice improving in the areas of tone production and easily attainable range extension within a few weeks, you’d better find another teacher – fast!
Many teachers give their students the same vocal problems which killed their own careers and made them teachers. Before studying with a teacher, ask for a simple demonstration of the teacher’s own ability – especially his/her ability to negotiate their own passage areas. Audition the teacher!