A part of listening to people is the ability to recognize facial expressions and body language, and interpret them properly. Now you are listening to more than just the words and the way they’re being said. If a person’s posture is out of line with what they are saying that’s important to know.
Try not to read too much into any particular posture or facial tick. Until you know somebody well enough, until you’ve observed for yourself in different situations, people can fool you. For example, deciding if somebody is lying by watching if their eyes go left instead of right is misleading.
Be aware of others’ body language. You can get better at it with practise and observation.
Test your skills here.