It is a grave problem who a student should trust as an English teacher. There are a lot of people who think they can teach English. But their English abilities vary. Some are competent enough to be a university professor, others don't even know the difference between comma and apostrophe. So it is up to students to decide whether to trust a teacher in front of them. But how can they do this correctly?
Fact: no teacher is perfect. Even a professor of a prestigious university can sometimes make a mistake. Once I heard a professor inadvertently pronounce the word "expertise." He pronounced the letter i like that in the word "time." Therefore, students are not advised to judge a teacher's reliability from her mistakes (unless, of course, they are not serious ones).
How, then, you may wonder, should students judge a teacher's reliability? In my opinion, the judgement should be based not on their ability but on their attitude. So observe a teacher when she makes a mistake or seems diffident. If she tries to cover it up or to proceed with the explanation in the face of lack of confidence, she should be judged to be untrustworthy. If the teacher admits to making a mistake or before going on with the explanation, says that she wants to consult a dictionary or a grammar, then she should be reliable. Although this is not meant to be an absolute test of a teacher’s reliability, I think it is a good one.