I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.
I think this misses the point of the OP, which wasn't that IQ or intelligence can accurately be guessed in a casual conversation, but rather that intelligence can be guessed more accurately than other important parameters such as "conscientiousness, benevolence, and loyalty", for which we don't have tools nearly as good as those we have for measuring IQ. The consequence of which being, since we can't assess these as methodically, people can fake them more easily, and this has negative social consequences.
On a second read, I agree with you- I don't think I paid much attention to the third sentence, because the first two both rubbed me the wrong way. I have known people who turned out to be all hat and no cattle, intelligence-wise, and see that as a general phenomenon, and think verbal ability can be very distinct from mathematical/technical ability. There's significant anecdotal and statistical evidence for that.
We have good measures o... (read more)