Are Recording Engineers "Certified"?

It often amazes me the differences in recording quality from one album to another. I would imagine that each and every studio, recording electronics system, and engineer would each have their own "sonic signature". But how is the recording engineer evaluated? Other than a possible degree or diploma from an accredited school, is there an auditory test that they are subjected to?

Does the record company just trust their credentials and put that engineer at a mixing board and say "Here, record this"? Is that engineer's hearing evaluated. Are they required to take annual hearing tests? Who is the person or group of persons, that give the final "thumbs up" to a recording?

And who decides if the final recorded product is of "acceptable sonic standards"?

Does a record company assign certain recording engineers to certain musical performers? George Martin for example, recorded classical performances for many years before recording The Beatles. I have seen Bob Ludwig's name on hundreds of recordings in a lot of different types of musical styles and disciplines of music. There are many notable recording engineers out there and obviously have been around long enough to have established their credentials.

I know that this a myriad of questions, but I think most of you understand the focus of these questions.