A teacher should be properly trained and certified by an accredited school or "master" instructor. Be wary of teachers who take crash courses.
Find a teacher who you like and connect with. The relationship between student and teacher should be one of trust.
Don't be shy - ask for a skills demonstration and proof of certification.
Find out how long the teacher has been practicing and how long they have been teaching their particular discipline. Also ask if the teacher participates in on-going education, workshops or trainings.