It is not possible to “manually” mark a movie or tv episode as “watched” (there is no “option” button menu selection for this) … it’s an automatic feature
But, you need to have the “Media Library” set to “ON” to enable the “Watched” icon
When using *Network Shares* you need to add them to “My Media Library” press the RED(B) button on your Remote Control.
Here is the result … (the Half Circle Icon aka. Watched Icon is now visable)