George,
If you are talking about the side mounted engine fan, once the engine gets to operating temperature, you can crawl under the coach to see if it is running. Alternately if your C12 runs around 185 most of the time when driving and maybe gets to 200 climbing steep grades, the fan is operating properly. No need to visually check it. The fan on your coach is run by a hydraulic pump and controlled by the wax valve which turns on the fan when the engine temp gets to a point where cooling is needed. Thus at start-up the fan won't be running.
Steve