If you know it about yourself (or about someone you know) that you (or they) are extrovert or introvert, what do you consider the most important sign of being one?

If you ever met someone and just could tell if they are extrovert or introvert, what behavior, sign, or body language made you think that? What do you think about being an extrovert or an introvert, and what do you consider as the biggest impact on someone's life that can change their behavior and way of thinking?