I start a timer on my phone and run it up to ~ 8 hours to mark the end of the day. While the timer is running, I don't do anything too leisurely (e.g., playing video games). If I want/need to do something unrelated to work, I stop the timer.

The idea isn't to work exactly 8 hours per day. It's to get a sense of how much of my time is going towards work-related activities.