One of the most underutilized skills we have as humans is that of calibration. This is where we look at a person over time and, without labeling the emotions, are able to identify different “internal states” by looking at the external cues.
People are great pattern matchers. We are so used to matching the patterns for various emotional states that we want to extend some of those cues into channels that don’t play well at communicating body language and tonal qualities. Know what I mean? 
LOL <G>
Calibration is different than looking at someone and saying “they look sad” or “they look happy” — it is working without emotional labels. We may still need to refer to those states, so we just do it without emotional labels by using identifiers like A and B.
One way to practice calibration is to have someone sit where you can see them and have them think of a person, we’ll call them person A. Once you think you’ve seen enough to be able to distinguish what physiology goes with person A, then, break their state, clear their “palette”, and then have them think of a different person, call them person B. Once you’ve got your impression of their person B state, the real fun begins.
Here is where you start asking them questions where the answer will be one or the other of the two people. You might ask which of them lives closer or which one is taller. They get to think of their answer without saying it out loud and you get to observe their physiology and say whether you think it is person A or person B. Now they get to confirm or deny your answer and you ask another question.
What does playing this game do for you? It gives you a chance to calibrate and be able to identify a given state without any expectations or preconceived notions about what that state “means.” Once you’ve practiced this in a safe environment, you can take those same skills out into the “real world” and apply them.
So, when would you like to be able to tell two different internal state apart? Maybe every time you’re wanting to influence someone’s decision for the better. If you’re selling something or dating someone (which is a form of salesmanship!), you could calibrate between good and poor decisions that other person has made in the past, then know which way they are currently thinking about decisions with *you*!
Of course, you can’t put them in a chair where you can see their reactions and ask them to “think of a good decision you made in the past” while sketching notes down on paper. Or can you? Isn’t that the sort of thing we do all the time and call it “small talk”?! And notice it wouldn’t take much to make that question work in casual conversation.
For more practical exercises like this one and to boost your ability to calibrate, get started with NLP Monthly now.