In my experience people don't have the ability to determine if someone is smarter than themselves. They can detect that someone is a different intelligence level. Most people will react the same to someone with much greater intelligence as someone with much less intelligence.

Secondly, intelligent people modulate their communication to be what the people they are talking to expect. They won't use big words at the bar, but will totally change in an academic or professional setting.

One way that "smarter" than oneself is revealed is when a person predicts or anticipates something. That could be experience, or just indicates that they had already processed the situation to the same level.