How do I know when I'm done with Her Story?

Solution 1:

As @TheForestAndTheTrees pointed out, you get a message from 'SB' once you're halfway done.

Once you've seen the credits at least once, you'll be given hints for two commands. One of these hidden commands unlocks a "program" that shows you a bunch of squares that represent which videos you've seen and which you haven't... and which order they appear in.

Once you unlock it, the icon for launching this program is hidden behind the video search window.

Even after you see 100% of the videos, there's still no real ending. Which makes sense as you're supposed to figure out what really happened on your own.

Solution 2:

After you've uncovered 50% of the videos, you'll get a message from 'SB', asking you whether you're done. If you enter Yes, you'll exit the game and get a very short reveal and the end credits.

If you entered No, you can recall 'SB' afterwards and say you're done again.