Looking for signs of …

Do you ever get the feeling people are not understanding what you are saying? What are some of the signs that this is happening to you? You are talking, but instead of people commenting, they are shaking their head with a stiff smile on their face as if they are agreeing. Another sign is no one contributing to your topic. There are many ways to tell when you are not heard. I would love to hear how other thoughts on this topic.