There are various ways to determine if a person is qualified to do a task or trade. Licensing through the state is probably one of the most common and is often required. For specialty trades, California has always been one of the more challenging places to get licensed in and—in an ironic way—one of the easiest.
Licensing is through a state agency called the Contractors State License Board. I am very familiar with the process. I have done it twice for the same license with about a 42 year separation. I first took and passed the exams for lath and plastering in 1980. Then in 2014, I wanted to re-instate my license and had to re-apply by submitting all new paperwork and pass the required exams again.