I'm not sure that any testing would be foolproof, or that any system which did prove foolproof would be acceptable to "authority". ATC people are among the looniest people I've ever met; my wife tells everyone "they're all raving mad". So what sort of system would or could quantify that strange personality trait?
IMHO educational qualifications mean very little after initial selection. I managed to scrape 2 GCE "O" level passes, neither in a science subject, but I worked alongside people with 6 A levels, two degrees, etc. I'd be the first to admit that my general intelligence is way, way below that of some of my ex-colleagues but, like them, I had the peculiar ability to do the job. I have seen many extremely well-educated guys fail - they simply could not hack the practical job, which requires a certain flair. Not only that, but controllers are often suited to one particular job and cannot do someone elses.. I could do the Heathrow Tower and Approach Radar jobs but was certain that I could never have managed Thames or Essex Radar - those people are of another world! I used to sit beside the TMA controllers and watch with awe - how on earth could they do it? Equally they'd come and watch the approach controllers, totally baffled as to how we sequenced traffic with such tight spacing. Very experienced instructors from The College have often failed to re-validate when posted back to an operational station.... And senior and experienced controllers might find College work impossible (me for a start).
From the many trainees I encountered during my career I have no doubt that those with a keen interest in aviation matters - yes, including spotters - go on to make good controllers. Their chance of success is enhanced if they are posted to a station of their choice. It's very depressing for a training officer meeting a trainee for the first time when he asks: "Did you want to come here?" to be told "not bl**dy likely". That signals bad problems. On the other hand, the trainee who views his posting as a dreamn come true (I did) and is eager to do the job to the best of his professional ability stands a very good chance of success.
Good luck to all who are going through the application process. Those who make it right through to validation are in for a truly fantastic career - probably the best job on earth.