Here are some relevant sites:

Find a Great Yoga Teacher: 5 Questions You Must Ask to Find a Qualified Yoga Teacher – Pretty straightforward. I include it because it might aid someone in beginning a dialogue on the matter.

American Yoga Association: How to Choose a Qualified Yoga Teacher – This guide takes into account many of the traditional qualities of yoga instructors.

What Is the Difference Between a CYT-Certified Yoga Teacher and RYT-Registered Yoga Teacher? – Some technical wording assistance.

Be Your Own Guru: Guidelines for Healthy Relationships with your Yoga Teacher/Guru – This is more for AFTER you have discovered an teacher. I believe it includes some really useful information (possibly even beyond student/teacher interactions.) Given the vulnerable nature of being a student, it is important your instructor is someone who fully deserves your trust and respect.