Regarding certification, I'm with Jason that it's not as useful for coaching on an individual level. As someone who works in a highly regulated industry, I understand that certification and licensing can establish that people have met the minimum requirements or competencies for a particular field. These fields are usually ones that involve public services or individual safety, such as health care, engineering/electrical codes, airlines, teaching, fire/rescue. Within these fields, two conditions appear to be important. The public often lacks the expertise to assess the competency of the professional, and an incompetent professional might cause great harm to people or property. In addition to certification and licensing, these fields are often highly regulated by law.
Certification cannot determine quality or excellence or appropriate fit of a professional to an individual. For example, many surgeons are certified, but vary highly in skill, training, and surgical success rate.
In ambiguous cases, referrals are good. Asking relevant questions is important. Also, look carefully at the background, past experiences, and accomplishments of the professional. The qualities of a good professional should be apparent upon interacting with them. For example, it's not recommended for people to put their MBA degree on business cards, as it's thought that "
the business skills attained via an M.B.A. should be evident in one's work or conversations". The same would appear to apply in finding a good coach.