Here’s a really good article from the Huffington Post about criteria for assessing a mindfulness teacher. I agree with all the author’s points and feel they are a good set of guidelines for determining whether or not to work with someone. One point I might add is whether or not the person attends regular intensive silent teacher-led retreats. Intensive practice is what really keeps mindfulness teachers learning and growing. I would say that at an absolute minimum, a teacher should attend at least one 7-day silent retreat a year.
http://www.huffingtonpost.com/deborah-schoeberlein/who-should-teach-mindfuln_b_7227540.html