Which may still appear to be just another form of intelligence tests (

Yes, I have a feeling that "capability of rationality" would be highly correlated with IQ.

Not completely reliably, but sufficiently that you would want your employees to be tested by that test instead of an IQ test

Your mention of employees raises another issue, which is who the test would be aimed at. When we first started discussing the issue, I had an (admittedly vague) idea in my head that the test could be for aspiring rationalists. i.e. that it could it be used to bust irrational lesswrong posters who are far less rational than they realize. It's arguably more of a challenge to come up with a test to smoke out the self-proclaimed paragon of rationality who has the advantage of careful study and who knows exactly what he is being tested for.

By analogy, consider the Crown-Marlow Social Desirability Scale, which has been described as a test which measures "the respondent's desire to exaggerate his own moral excellence and to present a socially desirable facade" Here is a sample question from the test:

1. T F I have never intensely disliked anyone

Probably the test works pretty well for your typical Joe or Jane Sixpack. But someone who is intelligent; who has studied up in this area; and who knows what's being tested will surely conceal his desire to exaggerate his moral excellence.

That said, having thought about it, I do think there is a decent chance that solid rationality tests will be developed. At least for subjects who are unprepared. One possibility is to measure reaction times as with "Project Implicit." Perhaps self-deception is more congnitively demanding than self-honesty and therefore a clever test might measure it. But you still might run into the problem of subconscious cheating.