Personal observation
Personal observation allows you to confirm for yourself the reports you're getting about the operators and the quality of the work they're producing. Maintaining a presence on the ground also helps you to convey the message that you are taking a personal interest in what's going on and aren't simply making decisions from a remote location.
If you regularly walk around the site, or even work side-by-side with the operators you're responsible for, they are far more likely to come to you with their concerns or suggestions for improvements.
But keep in mind that some workers tend to behave differently when 'the boss' is around, so you need to be careful not to let them give you a false impression while you're there.