Mouth: Disagreement also shows up in compressed or pursed lips, clenched jaw muscles, or a head turned slightly away, so eye contact becomes sidelong.

FORBES: The Body Language of Disengagement--and How to Deal With It

He tested his muscles, stroked his head thoughtfully, and felt the rest of his body: did he still have the strength for some powerful fame- and prize-winning action?

NEWYORKER: Among Animals and Plants

Typically, someone who is in agreement with you will smile and nod as you speak. (Disagreement shows up in compressed or pursed lips, clenched jaw muscles, or a head turned slightly away, so eye contact becomes sidelong.) But smiles are often used as a polite response and to cover up other emotions.

FORBES: Body Language Savvy for Sales

Now, sense other muscles on your face or head that are tense.

CNN: How to relax at work

The small dog came bounding through the grass, all tight muscles and yap, went for the shed, braked, stuck its head in the door and snarled, small body rigid with excitement.

NPR: Excerpt: 'The Broken Shore'

And the muscles in their neck, that on Earth work all day to support the head, are weak, so they can easily sprain their neck by looking to the side, too quickly.

FORBES: What's It Like Experiencing Gravity After Being In Space For 6 Months?