You should start with the knowledge that, generally, best practices include a tooling setup chart, a tool path print, set-up procedure, operator instruction and PM procedure. If you walk up to a machine, and you didn't build it and didn't program it, how would you know that stuff?
In order to have efficient set-ups (reduced set-up time) and consistent start-ups, you should have a set-up procedure (speeds, feeds, coolant line location, coolant flow, which coolant used, etc.)
You should have an operator instruction to understand what corrections to make while running the part - such as: If a particular dimension starts to get surface finish problems than first try changing the tool. If that does not work, then the material lot is different and the spindle speed or the tool travel speed needs changed.
You should also have PM instructions to know when and what to maintain on the machine. Some people claim thy follow the manufacturer's suggested PM, but when you check what they have done versus the recommended PM, they start to qualify their stand.
Now, if one can explain how the operator knows this information intuitively from their training, then you can start to ask the question of whether any of the above work instructions are necessary. That means so intuitive that if your set-up man goes on vacation or leaves altogether, you can keep on plugging and chugging with little problem. The complexity of the work being done on the CNC machine has an impact of the degree of documentation. If you are doing essentially the same work that could be done on a drill press, then that will not require the same detail as a 10 tool set-up with a 20 minute process time.
