A quick little sign to watch for when conversing with someone is the Lip Pull, which is when a person not only purses their lips but pulls them quickly off to one side of the face. This is a quite dramatic facial gesture, readily seen by others.
Typically this is a quick gesture that lasts for just an instant, though it could be held a couple of seconds. The lip pull tends to mean that the person dislikes or disagrees with what is happening or being said. It can therefore be a very telling gesture, letting you know when someone is unhappy, even when they are trying to hide it from you!