If we need to be satisfied in our interactions with other people, we need to observe. From my perspective, this includes listening to whatever they have to say and also paying attention to their body language, including the tone of their voice. Words never express the whole of what they have to say, simply because they can’t (you can read here my post about words).
The non-verbal elements can say much more because they are not consciously controlled so you can learn things about a person that he/she doesn’t wish to say. For example, changing the tone of his/hers voice into a higher pitch can express excitement caused by either the surprise of the moment or by a strong inner feeling about something from that moment (like answering to a phone call from his/hers crush). Another example is the distance between people. If you’re noticing someone keeping a certain distance to people, usually the same distance for most people, but then you see that he/she gets closer to someone, it’s most likely that he/she likes that person. Same principle applies to people that we don’t like. We usually want to keep a greater distance towards that person.
I think that this is an art because it takes a great power of concentration. In the same time, you need to pay attention to the words, the voice tone, the body position and to all the elements that might cause those. And after all of those observations, you need to spend some time to process all of that. Why would I do this?, you might ask. Well, it depends of your interests. If you give a damn about those people, you would want to know them better.
I’ll give you a practical example. Let’s say you want to interact more to a certain person, but that person is very quiet. At a certain time, you notice that person having a great conversation with someone else on a certain topic. There are two hypotheses here: either that person likes the other one, or he/she likes the topic. Keep paying attention to that person’s interactions and if you notice him/her talking to someone else about the same topic, it means that he/she likes the topic, but if you notice him/her talking to the same person on different topics, it means that he/she likes that person. Also, you need to pay attention to physical contact (it’s zero distance, so it clearly means liking the other person aka the touched one).
This art can create knowledge and knowledge can create frustration, so don’t ask questions if you’re not prepared to know the answers.