I have two methods that I use, depending on the circumstances.
One is to make another folder called "Watched" and dump all movies I have watched into it.
The problem with that is that when I am copying movies to my back up drives, I would have to go through the whole list there and put them in a "Watched" folder there too, or I find myself with duplicates.
The other method is to add a letter or number to the name.
So I hit rename and generally add a capital "X" at the end of the title, so that the movie retains its alphabetical place, but again the problem of duplicates will occur when backing up those movies to an external hard drive.
I've been watching this thread, hoping for a better solution, something like what you mentioned, where you could just check mark them or change their icon colour, or anything that would be simple to do that would make the watched ones look different from the un-watched. I feel that my two ways of doing it is rather pitiful and too complicated with regards to avoiding duplicates.
I hope someone suggests a better solution
All is well in Paradise.