Updated June 10th, 2019.
When you make a change to the Screen Saver options in the Mac’s System Preferences / Desktop & Screen Saver preference pane, you’d naturally like to test things. Problem is, there’s no Preview button… unless you know how to make it show up. The trick is you have to move the mouse over the box that looks like a little screen (outlined in red below). When you do that, a Preview button appears. All you have to do is click it. See the arrow.