What is a sign that someone is intelligent?

Half of me wants to say "there are lots of signs, depending on what kind of intelligence you're looking for", and half of me wants to say "intelligence isn't well-defined, so there's no point in looking".

In either case, I suspect that intelligence is probably not the signal you're truly looking for.

If you're worried about others' intelligence, be more specific. Are you looking for quick learning ability? Are you looking for people that have strong social awareness? Why does it matter?

If you're insecure about your own intelligence, find joy in testing the limits of your mind! Do difficult things. Create masterpieces. Explore all the knowledge this wonderful world has to offer.

Worrying about "intelligence" is like worrying about "athleticism". I think it's more fruitful to focus on actually doing things than thinking about the stats required to do them :)

Then there's crows and ravens... https://www.sciencealert.com/crows-ravens-corvids-best-birds...

In the movie Men In Black there's a scene where Will Smith's character is applying to become a MIB agent. All the applicants are required to fill in a form and all have difficulty doing so. He is the one who resolves the problem by dragging a low table over to his position to enable him to complete the form easily.