The first thing to check is that your NightShift device is online. You can see this by clicking on the info button and looking at the top of the panel that pops out.
Next you can check if NightShift is serving your content by looking at the playback list.
Here, you can see that Long Shadow was played. The “Netflix Played” indicates that this show was NOT served by NightShift. It says “NightShift Played” when the content was served by NightShift.
When Netflix content is being actively watched, a small “Now Playing” panel will appear. This also indicates if the content is being served by NightShift or coming directly from the Internet.