How can I find out when the computer monitor's inactivity timeout is triggered and what process triggers it?

I want to use [slock](https://tools.suckless.org/slock/) to lock the screen when the screen's inactivity timeout switches the monitor off or into standby and want to know what process triggers it, or how it is notified to the system. Is there a way of checking if it is triggered by the monitor itself or triggered by the display system? I suppose that if the monitor's timeout period is lower than the one set in the operating system it will trigger the standby before.