Building a Mental Model of Performance

How do you build a mental model of how somebody is performing? What does it look like. How do you start building that mental model? So this is a generalization, but today, people generally are very, very good at the one thing they do. That's the first starting point: can somebody do their job? And then is ther additional training or experience that they need to get? And then beyond that, then it's how do they work with others? And then what are the other skills?