I am wrench in an LBS.
Certified isn't the issue, learning how to work on bikes is. The best way to do that is to learn from an accomplished mechanic.
There is no, as far as I know, national test that certifies bike mechanics. There are two schools that I know of - the United Bicycle Institute in Ashland, Oregon and Barnett Bicycle Institute in Colorado. Both are very good.
But, people who have attended both has told me that you can learn just as well by working in an LBS. Most mechanics learn by essentially being apprentices. They start out sweeping and cutting up cardboard boxes and then step-by-step learn to become mechanics. It takes awhile.