I think this is a very valuable skill for instructors to cultivate. Deploying this skill effectively requires getting to class early and being prepared before students arrive. Being prepared allows you to greet students and assess their body language and reactions. Soon into a class you can pick up on changes. From the way "Suzie" comes into the class you can sometimes know whether she will need some extra attention, or perhaps to be let alone for a while. The key is getting to know your students. What example can you share of how successfully monitoring students has helped you in one of your classes?