Under Windows: use the color scheme, if your screen is blue: reboot and watch your clock.
If your screen is black: reboot, watch your clock, and think to sleep.
If your screen is always black even after and during the reboot: reinstall the monster and sleep.

Under Linux: There is not any such indicator, drink a lot of coffee and good luck...

More seriously, I stop when I can't view correctly my screen or when my rock cd stop and that I realize the entire house is absolutely quiet.