Depends upon the campground.
Generally you just have to drive around and look for open sites. The 'ranger' when you check-in can give you an idea where open sites might be. Often on Friday there will be two or three folks driving around looking for a site at certain times.
As far as leaving for the day with a small rig - you get a piece of paper when you check-in which has your departurer date in big numbers. It goes either on a window of the camper so that it is visible from the road, or clipped to the power pedestal. If you see one of those on a power pedestal, you don't camp there.
The other thing is you look for 'Personal Site' signs (or Annual Site) so you know those sites are not available to use. If you see an empty site with a brick patio, a small storage shed, etc. - that is obviously a site that is rented for the season and not available.
At some campgrounds, if you see an electric meter on the site - it is not availalbe for use except by the person paying for the electricity.
At one CG north of Dallas, you look for a 50Amp site with a padlock on the electrical connection. Since they charge $3 per day extra for 50Amp - a padlocked pedestal is available. One without a padlock is rented.
It is easier than it sounds to find a site, just you will almost always see a great site you would like to have, but can't.