Configurable as of PMS 1.30.1.6562. 90% in earlier versions.
Settings → Library → Video played threshold
70% to 95% in 5% increments.
Plex added credits detection in 1.31.0.
You can also choose to have media marked as watched when the credits are reached.
Settings → Library → Video play completion behaviour
Note that credits detection is enabled by default and uses the CPU. The feature can be controlled at the server, library, or movie/show level. Suggest you read the support document first (and don’t be surprised when the server CPU spikes).
With Tautulli you can list the play history for each user.
