Here are some tips I have picked up along the way to read the person you are talking to have found them especially useful running an organization that is distributed and virtual. We conduct most of our discussions through video on Google Hangouts, and reading body language and facial microexpressions are a key means to reading nonverbal signals.
When talking to one person
Read expressions and gestures in clusters. The look of critical evaluation when combined with closed body language, crossed arms and legs, indicate that you may need to make a more convincing argument.
Talking to many people
The bigger the audience, the bigger the gestures need to be.
Use wide expansive, open gestures. Don't cross your legs or arms. That
bars the audience and shuts them off. Stand confidently and welcome the
audience to listen to you. Ben Horowitz does a great job illustrating this at our Technovation keynote a few years ago.
Dr. Paul Ekman is the pioneer in this field. His book Emotions Revealed: Recognizing Faces and Feelings to Improve Communication and Emotional Life is a great starting point.
The six basic emotions are shown below. If you watch closely, you will see various (and fleeting) manifestations in people's faces. They will give you a very accurate window into what your listener is thinking of what you are saying.
Listen. Observe. Talk less.
"The reality of the other person lies not in what he reveals to you,
But what he cannot reveal to you.
Therefore, if you would understand him,
Listen not to what he says,
But rather to what he does not say"
― Kahlil Gibran