Here's a good way to spot someone who's smart and engaged. When the conversation turns to something they don't understand, when there's a term or concept that's unfamiliar to them, that person doesn't sit there and nod his head. He doesn't pretend to understand when he doesn't. He doesn't disengage and become less interested just because he's no longer following. He confidently and politely says,
"I'm sorry. I was with you until just a second ago. What does that mean?"
High school teaches you to believe that you should always know the answer. When you're doing a problem in trig, answering a question on the SAT, or being called on by your Spanish teacher and you don't know the answer, it's bad. There are points deducted and penalties to pay.
But here's the thing about smart people–they don't always know the answer. Nobody does. And how you handle yourself at those times says a lot about you and your desire to learn.