I just walked past a car advertising for a Certified Classical Homeopathist (as opposed to the dreaded new school homeopath bastards) and thought it must be pretty easy to say you are "certified" at something. So, what about DMs?
If you were in Human Resources at a fabulous Role Playing Resort (you know they're coming) what qualifications would you want from potential DM hires?
I'm guessing you'll want to see them in action. Maybe throw a set adventure at them, that they can create objects and handouts for, and then for a different session force them to improv completely?
I know for myself that every session seems to be a first experience with something: first sea travel, first splitting of the party, first urban adventure, first heist, etc. I still haven't got a lot of mid level character experience, or high level character experience.
Maybe the resort would have beginner players go with a newer DM. Or, actually, you'd probably want the opposite: your most experienced DMs handling player first experiences.
If there was a qualifying exam to become a Certified Dungeon Master, what would you want it to test, and here's the kicker for those of you experienced with Assessment, how would you know if they did well?