My signal is usually someone's beliefs on certain controversial issues that arouse emotional gut responses in most people.
E.g., someone's view of the war on drugs is a pretty good signal of how rational they are - to a limited extent (although opposing it doesn't mean that they'll have Razib Khan or Robin Hanson levels of rationality). Mostly though, it just filters out irrational people (and it does a better job at filtering out irrational people than, say, views on abortion).
I do generally investigate more deeply though - beliefs on the public education system is another measure I use (and one where most academics, unfortunately, really fail on rationality). On that, at least, the "rational" position seems to be supported by a small minority of the population (other possible signals: views on cryonics, the use of mind-enhancing drugs, eugenics - this is actually a strong one, and life-extension)
I also use irreverence as a signal too. If you put too much respect on someone, then you'll get offended if someone points out possible defects in that person's way of thinking. Every statement someone makes must be critically evaluated, and yes, this applies to Razib Khan and Robin Hanson as much as anyone else (even though I highly respect them)
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(of course, you also have to consider their "potential" for being rational - even I used to buy into the BS of the war on drugs and increased support for public education in the way it's structured now)
A rational reason for not professing opposition to the war on drugs is that you do not want to lose status. Surely your tests are for identifying contrarians?
These two questions seem to be somewhat interesting, especially for those interested in rationality outreach.
What makes someone more likely to study rationality?
More likely to become a higher level rationalist?
A few thoughts:
Empirically, rationalists seem to be more into technical fields than average, and more interested in an explicit understanding of social things than most technical people.
People who can more clearly see deficiencies in themselves, and who try to solve problems seem more likely to become rationalists, when exposed to rationality.
People who are motivated to pursue rationality for instrumental goals, rather than for funsies, seem to become better rationalists.