In one national park where we volunteered, our last campground round was at 10pm to make sure noise was minimal, etc. At that time we also checked the car/post tags to make sure the dates jivved.
The next morning we made rounds at 6am. If someone was at a site without a car tag or post tag we left a note on a form for them to come into the office at 8am when it opened and to pay. On that form was a spot for their license number. We rarely had anyone that didn't pay because they know we had their license # and yes, they would be fined - and it would be way more than a campsite charge.