Bryan February 2016
This command will output everything relating to power for the device:
adb shell dumpsys power
You can pipe this to a grep to get the values of
adb shell dumpsys power | grep 'mHolding'
If both are false, the display is off.
mHoldingWakeLockSuspendBlocker is false, and
mHoldingDisplaySuspendBlocker is true, the display is on, but locked.
If both are true, the display is on.