Most of us aren't psychic, we are not trained to be mind readers, but  especially during negotiations we would love to know the thoughts of our opponent.
And in some ways we can by reading the body language.
Because even the most experienced character who is trained to mask their expressions may still betray themselves with their bodies.
There are nonverbal cues from the unconscious, out of control of our conscious will.
And there are other signs we give by accident because we don't know that we tell more than we intended.
That is why you want to watch for signs of deception.

Cues of deception can be movements such as rubbing the side of the nose, covering of the mouth with the hands, jerking the head quickly to the side or leaning away from you.
The cues are even more significant, if they occur while your opponent is saying something critical to the negotiation. 
Have in mind; these behaviors may simply result from nervous tics, but they can also expose a liar.
To read the body language accurately watch the signs in clusters and in relation to the context.

Learn how to ask the right questions and to understand the nonverbal cues you get as part of the response.
Do they match what they say?
Most people want to subconsciously apologize for a lie.
Because they feel guilty for lying many signs can be detected in their nonverbal behavior.

Generally, if someone is lying they will not look you in the eye, at least during a certain part of the conversation.
Naturally, we make eye contact for at least half of a conversation.
Anything less than this could be suspicious.
But be careful; some people are so shy that they avoid eye contact with you and there are others who are lying who learned to look you in your eyes, to make you think they're being honest.

To know that will help you to negotiate better.
You will know if you already got the best offer and if the other is saying what is on his mind right now.

Beware of interpreting every single little sign of your opponent.
Resist drawing hasty conclusions based on someone who suddenly starts scratching the neck or touching the nose.
As previously mentioned; read the body language in clusters and context. This will help you to gain insights as to how a person feels.
Learn how to include the full range of expressions and movements in your observation.
The movements of the human body include the eyes, head, shoulders, lips, eyebrows, neck, legs, arms, fingers, hands and gestures.
The risk of reading your opponent is that you concentrate too much on just one single cue.