I can set my monitor off with
xset dpms force off but something prevents KDE from setting it off. On the other hand, when I play a long youtube video it might go off (!). This correctly happens after 5 minutes, as I have already set in "System Settings > Power Management > Energy Saving > (checked) Screen Energy Saving > Switch off after 5 min".
How to check which activity/program is preventing my monitor from going off?