Licensing probably could work in this industry but it would be a tough nut to implement. For starters, as others have mentioned, with so much change happening, re-licensing would probably be needed, and of course that's a huge cost, a pain, and overhead as well. But then again, as I think about potential solutions, I am kind of coming up dry. Really need to mull on this some more.
It can just be a general aptitude test done in several popular languages of your choice with some level of customization based on what sort of programmer you want to be.