Choose an area of the language you have problems with. Ask for a free sample lesson and ask the teacher to teach you the part you have problems with. If after the lesson you realize you have no problems, that everything has dawned on you, that you feel you have found a new horizon, he's probably a good teacher. You can also ask him the following questions. Ask for details and clarification and watch his face to see if he really knows what he's talking about. 1. I hear a lot about contrastive analysis, but I don't understand it quite well. Can you please explain it to me? (If he explains that well, then you can go to the next question.) 2. What are the major differences between my language and yours? 3. What does phoneme mean? I really don't understand it. 4. (Assuming you are learning English) I never understand when I should use past simple and when present perfect. Can you tell me the difference? ( If you're learning another language, you should find something similar). 5. (Again for learning English) Can you tell me the difference between first and second conditionals? If you are clever enough, you can find other questions to ask. If you have taken language lessons before, you know which aspects of the language your teachers usually have problems teaching! Ask the new teacher to teach you those aspects. And one other great way to tell a good teacher is this: Teach him your language for a while. See how good he is at learning your language. If he's a good student, chances are he's going to be a great teacher. And a final note: Once you have chosen your teacher, trust him and let him direct you through the course. Don't write lesson plans for him;) Good luck! * Whenever I use "he" it means he/she/it!!

By assessing his/her performance.

I agree with Dark panther. Their reviews will tell you.

Look at their reviews!

if ur english get improved with a human manner of his/her teaching then he/she is good teacher otherwise not!!