How to Assess Teacher Trainings
I would further caution you against courses assembled of many guest teachers, because this can be confusing. If the teachers are not coordinating the course but are simply presenting their own separate materials, the course will likely lack cohesion. (The exception might be guest presenters on the subjects of pPranayama, anatomy, or philosophy.) You are better off with a course that is based on one method, or a course whose teachers work together regularly and have formed a unified presentation.
What should you look for in a teacher training course?
Examine the key instructors. A course is only as good as the teachers teaching it, so opt for courses taught by senior teachers. The instructors should have at least 15 years of teaching experience, and they should be generous with information, not constantly asking you to sign up for more courses.
It may not be necessary to like the trainers. I know this may seem radical, but what you need are teachers who know yoga alignment and philosophy. In other words, your initial training should emphasize the correct teaching tools that you must have to be a good and safe teacher. After that, you can go to teachers you like in order to improve your personal style of teaching.
Interview the teacher by taking a class with them prior to signing up for the course. See if they will set up a time with you to discuss the course curriculum.
It's a good sign if a course requires an application or some years of practice before admitting you, particularly if they want to see you practice beforehand. Courses that do not require applications may admit students at a huge variety of levels, and you may be underserved.
Look for courses that emphasize alignment. A teacher training should not be a relaxing vacation; it should be rigorous in practice and information. Check over the manual and other curriculum materials before signing up—they will tell you a lot.
I should add that many schools like to train the teachers they hire. You may want to look at where you want to teach and include that school's courses among the ones you choose to participate in.
Gyms are more likely to care about certificates, but often the director will still watch you teach. I believe that a training completion letter or proof of attendance will often suffice, but building a good resume, listing workshops and several teacher training courses, is more impressive.
I personally like the Iyengar teacher training certification. In order to become certified, you must teach in front of a qualified trainer, who will give you feedback. It seems that the Iyengar association is truly interested in turning out qualified teachers, not just in making money. The only problem is that many of these trainings ask you to sign a commitment to teach only Iyengar Yoga.
We live in an interesting time in the yoga world. The business is growing quickly, and many teachers are becoming popular based on personality, not experience. I want to thank you for your question. It is a solid one and raises important issues to consider as we look at the increasing popularity of yoga today.