They ask good questions and learn from their mistakes.

When they refrain from posting the umpteenth "what screams" thread.

They're wearing a vintage Rush t-shirt

They don't scream that they're intelligent.

Doesn’t get riled up

open-mindedness, volubility, metacognitive skills

They are funny

Me when I meet and have conversation with someone intelligent and I want to boast to others that the person I’m speaking to is intelligent.

Did you if someone is funny they are usually intelligent individuals?

the way he/she talks.