They turn off quickly. Effective listeners separate the wheat from the chaff. They choose to listen to discover new knowledge. Concentrate on the words and message, not on the professor's looks, clothes or delivery. Poor listeners notice faults in a lecturer's appearance or delivery.
Effective listeners strive to pick every professor's brain for self-gain.
When you hear something you're not sure you agree with, react slowly and thoughtfully. Poor listeners stop listening to the speaker and start listening to themselves. They either passively reject what is being said or launch into impassioned rebuttals. When they disagree, effective listeners don't disengage or jump to conclusions. They keep their conclusions very tentative while getting more information.