Thanks for asking! Since Screencastify does not host videos (all videos are hosted on your Google Drive), and there is not a feature built into Google Drive that allows you to see how many times a video has been viewed, there is not a way to separate videos that you have watched from those you have not yet seen.
If you ever have any great suggestions like this for the Google team, please feel free to use the Feedback system that goes straight to the development team: https://www.google.com/tools/feedback/intl/en/learnmore.html