All doctors and dental employees must be licensed in their profession, especially in the USA. However, the procedures for practicing this profession differ from country to country.
Nevertheless, some dentists practice without authorization, especially in poor or low-educated environments, because they take advantage of people’s naivety, thus causing serious health problems.
Before going to a dentist you should check his educational background and his license. Moreover, his authorization could be removed due to ethical issues and fraud. So, pay attention to that as well!