Observing visual cues are a very important aspect regarding our perceptions and observations about our world. We all probably have some level of skill observing people's body language however some of us have more skill than others. Anecdotally, I find those who lack that specific awareness as probably the most capable of annoying others (or myself).
When some one's posture is stiff or rigid it can mean many things including someone who might be experiencing pain or discomfort or perhaps feeling some level of frustration or tension. Further clues would include facial grimace or a tight clench of the fist. Maybe it's time to run in the other direction but first look at the eyes because that can often be the telltale sign of reading the emotion or by looking at the person's facial expression.
It doesn't seem overly difficult to be able to read another person's facial expression or perhaps my experience or practice makes me more proficient in that area. Part of this involves eye contact. I get very nervous with someone who doesn't make eye contact with me. What are they trying to hide? Self conscious people have difficulty making eye contact and this can often be misread as rudeness or disinterest. Some people also believe that when people are not being truthful they are not able to maintain eye contact. A good sociopath probably can fool us though.
When you are speaking with someone you can usually tell how interested they are by how they maintain eye contact, nod their head, or hold a stationary position. I've witnessed people walk away as you are trying to talk and engage them and if this happens it's fairly certain they are not interested in what you are saying so stop trying.
People are not that difficult to read if you take the time to observe their behavior. Of course you can make some wrong guesses but it's also possible you might have some success at surmising a person's mood or intent. Isn't that sort of true of animals too? When a dog wags its tale doesn't that mean he's probably friendly? Unfortunately people do not have a tail to wag and don't usually bite.