Choosing a language teacher is a very personal choice. It is a question of “chemistry.” A teacher might be well-qualified, but still might not be right for you. That is one reason why I offer all new students a free first meeting: Only you can decide if I am the right teacher for you.
Only study with a native speaker! There are nuances and fine points and instincts that only a native speaker has. When choosing a native speaker, avoid someone with a very strong regional accent (like Scottish or Mississippi!) When you are studying a foreign language, you are also studying the country. A language is more than words: It is a style of communicating. To learn American English properly, you must interact with a real American!
Do you like the way your teacher talks? Does their English sound beautiful to you? Do they have a nice voice? Is that how you want to sound? Listen carefully and decide.
Your teacher should be able to analyze your English problems and have a clear plan. Your teacher must always be on time and prepared.
As an adult learner, you need a teacher who does not only teach “it” (meaning English), but who will teach YOU. You need someone who will analyze your English skills, your problems and your goals and create a program just for you. People often ask me which textbook I use. I say: “The student is the textbook.” How I teach depends on what that student needs and the individual way that student learns. From there, I choose teaching materials.
Your English teacher should have respect for your own culture and language. It is better if he or she knows a little of your language: That often gives a clue to your problems. In working with my students, I am always glad I have studied 12 languages. I can usually find some example that helps the lesson.
Really, the only right way to choose an English teacher is to take a few lessons with someone and decide if you enjoy them and whether you are learning something. Who is the right English teacher for you? Only YOU can answer that!