GetDevicePowerState sometimes works for monitors. If it's present, you can open the \.\LCD device. Close it immediately after you've finished with it.
Essentially, you're out of luck -- there is no reliable way to detect the monitor power state, short of writing a device driver and filtering all of the power IRPs up and down the display driver chain. And that's not very reliable either.
De uma postagem do StackOverflow encontrada aqui .