Aside from the inevitable Pilot bias, I wonder what the Test Battery would be for ATC? Talk to 6+ people at once whilst keeping track of all their responses, via RT and Landline, whilst keeping visual perception of where 4+ of them are located and the spacial risks they may be facing?
In the early 90s there was a problem with the aptitude tests for Fighter controllers (specifically what are now called Weapons Controllers) OC SFC at Boulmer said that he just needed them in the bar for a night or 2 and he would be able to grade them.
It didn't quite work like that but there was a short visit and some time on a console (+ time in the bar) - it did improve the pass rate.