I think the blindfolded obstacle course sounds like the way to go, partially because it would be extremely entertaining to watch.
As far as I know (I haven't tried out before, although I may this year), we just hold driver's tests. Whoever is the best at driving gets that job, and whoever is best at auxiliary gets that job. Of course, the mentors have to approve, so that makes sure that we get someone who has a good understanding of the game, knows strategy, has good communication skills, and performs well under pressure.