Real communication consists of information flowing in two directions. The obvious direction being "out," the other direction is "in." "Incoming" information is often referred to as "listening," which can be literally listening as well as reading, watching, etc. If you aren't listening, you are not actually communicating. Even if you are speaking/talking/writing, if there is no input/listening, you are not communicating, you are broadcasting.
It is important, then, to know when you are not listening. The first and most obvious sign that you are not listening is that you are talking. This, however, is not the only time that you aren't listening.<--break-->
Here is an incomplete list of indicators that you are not listening:
All of the above examples apply to when you are reading and watching as well as listening.
Listening is the single most important function for communicating and learning. If you want to become a better communicator, become a better listener.