They draw wisdom from multiple sources. Wait but that might be more wise than intelligent... But I guess those two tend to be seen together a lot.
They can switch up the way they talk to match the person they're talking to without sounding condescending. They listen to how others learn and explain it in that person's language of understanding.
When they explain something they make you feel intelligent. Explaining something complex in simple terms. Knowledge is so vast that they can talk about in the easiest way for someone else to understand.
They spend time thinking before asking a question.
They can genuinely consider an idea which opposes their worldview without necessarily accepting it.
They are curious about everything. To be intelligent you need to be knowledgeable and you cant be knowledgeable if you are never curious.