Observation. Mostly his inability to elucidate his speeches, especially his extemporaneous ones.

Also, history as a straight-C student. (I will admit that my standards for public office are higher than those for the average person.) Consistent business failures. Apparent inability or lack of desire to comprehend the other side of an argument (though this could conceivably be more closely related to sociopathy).

Actually, the thing that bothers me the most is (and this is a totally subjective observation) his pride in his ignorance. I absolutely cannot point to a specific episode that points this out, but I've known a good number of people who have this personality trait and there are unquantifiable (by me, at any rate) parallels between them and him.

At least he hasn't demonstrated an inability to spell. (Edit: Oh, but if he says ``new-cya-ler'' one more time, my head my explode.)