It's an important part of our socialization as humans to be able to read, interpret, and understand each others expressions. Even a vaguely perceptive person should be able to tell the different between someone being happy or angry from a familiar display of facial movements. Mind reading may not exist as such, but some people are able to seemingly read people's minds through microexpressions - the sort of involuntary reactions we have, even for a second or less, that betray our true feelings.
Instinctively, we may be able to figure out basic emotions (and whether they are genuine - such as whether or not a person is "smiling with their eyes"), but what about when people are fighting or masking what they truly feel? Turns out we still can't help but give clues. Becoming fluent in expressions is the basis of the TV show Lie to Me, in which Dr. Cal Lightman (played by Tim Roth) and his team assist in investigations through applied psychology.
You can also see just where you fall in the range of perception with this interesting facial expression quiz. How well did you do? Which ones had you stumped?