Usually for more wide-spread styles, the first step is to look up certification training courses in your area through the Yoga Alliance (http://www.yogaalliance.org) or the Yoga Alliance America. I know that in New York and in Los Angeles the Yoga Alliance is more or less considered "official." It’s not totally unlike the "certified organic" food (versus food that’s just grown without pesticides). Through the Yoga Alliance you’d be nationally recognized as a "certified yoga" instructor.

On a side note, certification standards is also an issue of controversy within the national yoga community, at least in deciding who gets to design what the standards are and how. In my own experience, yoga teachers are usually trained by their master teachers. This has the added benefit of taking-on the reputation of the master teacher, boosting your professional networking.