You can test whether your Ad Unit is functioning on your site by activating the Test Ad Unit function on the upper, right-hand corner of the Dashboard or Ad Unit sections.
By activating Test Ad Unit, correctly embedded Ad Units will produce a demo ad on your Sites for a few minutes before automatically shutting off.
You can enable testing on both desktop and mobile browsers.
If no video appears, try refreshing the page and confirm the compatibility with your site's platform (Wordpress, etc) or Ad Manager (DFP).
If you don't see any video yet, the Ad Unit code could be wrong (check for typos) or was embedded in a wrong location outside of the <body> tag of your page.
The demo advertisements will neither generate revenue nor show any video to the visitors of your site.
You can enable the Test Ad Unit to preview Ad Unit placement prior to your site being approved.