I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be"
No-one seems to be questioning this, but it conflicts with my experience as I often overestimate people's intelligence. This may be a failure mode confined to me, but I often meet and talk with people, who I'll "code" as being quite intelligent because they're well-spoken and broadly knowledgeable, (and tall and attractive and popular and hello-there-Halo-Effect). I'll then have to reassess this at a later point, generally by seeing them "in action" trying to reason about something.
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.