IMO it is hard to certify something such as CNC Operator, especially where the definition of operator varies from place to place. In one place it can be a warm body, in others it can be the ability to setup, edit a program and operate the machine.
To me an Operator is an entry level position, someone who knows or can be trained to operate the machine, perform operator checks, load, unload, check parts and input wear offsets. Where the person and their work ethic is more important than experience. When the person is able to operate and maintain the machinery with little or no supervision, do their own setups and hold close tolerances they are a setup person or setup/operator.
CNC Operator Proficiency Test
Black Dragon covered this, if you can't answer the questions you're not qualified to evaluate an applicant. Some of the questions are designed to present opportunities for discussion with the applicant.
Tom