Someone's eyes will give you a close insight into what they are currently thinking. The expression "the eyes are the window to the soul" rings true. In most people, whether they realize it or not, their eyes will subtly change depending on their mood. If you pay attention, you will notice that when someone is happy their eyes have a brighter color and tend to almost glisten whereas when they're sad or angry they tend to be darker and dull. Personally, I can figure out what someone has on their mind off of that alone but for those of those of you who need a few more clues, keep reading.
This is probably the easiest way to gather an idea of what someone might be thinking. In order to pick up on physical clues and cues you have to be close with the other person and be aware of their tendencies. For example, you have to know things like, how they normally stand, walk, what they do with their hands etc. We all know someone that walks faster when they are stressed or tenses their shoulders when they are worried. For me, I think of a friend who I noticed would walk slower and slouch when something was on his mind, or when he was tasked with something would walk fast and lean into his strides. Physical expression is a great indication of mood but does require some prior knowledge of the person's tendencies and physical traits.
When you are considering background evidence you simply take what you already learned about the person and apply it to how they're currently acting. If we take the example of my same friend who slouches and walks slowly when something is on his mind, I can extrapolate out what could be bothering him specifically based not only on his physical expression but also on my knowledge of what is going on in his life that could cause him to be preoccupied and stressed. People do not act "randomly" and rather than making sweeping statements based on actions alone it is important to consider the "why" that we find in background evidence.