That depends.
The criteria and definition of "certified" varies: certification can be given at the dealership level, from classes taken online or at a formal training facility, and certification can be from national organizations, like ASE, that require a minimum number of years before a tech can even test. Plus there are numerous areas to be certified in.
ASE | Home
Every tech's qualifications may not be certified, either. But you can and should ask to see their credentials.