How can I mark (movie) files in finder (as watched)? In my movie folder I have loads of movies. The ones I watched I want to mark as watched. 
I suppose I could either use the colors (like green for watched) or the Spotlight tags, but I would rather have a checkbox of sorts which I could tick.
The colors and the tags I don't find all that useable. Also, I do not want to add the files to iTunes. 

Is there something like this?
Simply put, No. Finder is for file management, therefore such feature will be weird. You might be able to find plugins for this, but I doubt it. Most people use iTunes to achieve such job, which you seems to be reluctant to do.

You can try making a separate iTunes library for these files, and not importing them to your new library. That might help.