Visually scanning a huge set of video thumbnails to see which have the Played check mark and which don't is a tedious task indeed.
Ideally there ought to be a Filter option to filter and display only played or unplayed files.
If that is difficult to implement, it might be quicker and easier to add a new Played (or Played status) option to the existing Sort menu, to sort by played status such that all played and unplayed files are respectively grouped together (in reverse order VLC can show the unplayed group first and then the played one).