Many states have no certification at all. Some are trying. New Jersey, Washington, Minnesota, Oregon, Virginia, New Mexico, Maryland and Utah have formed a consortium to pool resources for establishing standards and testing. The National Center for State Courts has aided these efforts by hosting brainstorming sessions and publishing a how-to guide that contains, among other things, a model certification statute and a wealth of practical information about managing court interpreting. The value of interpreter certification, moreover, depends on the validity of the testing methodology and other certification requirements.
Some states use court interpreter examinations that are of dubious validity.
Federal certification is usually a good indicator of competence. State certification may or may not be a reliable indicator, depending on the state in question. It is a good sign if a translator you are considering is accredited by the American Translators Association in the appropriate language combination. Reliable certification is one of the most important issues of the day. First, know your languages.