How a laptop "sees" external monitor via VGA cable?

Question

Iron

Faced such a thing: they brought a laptop with a screen hit on the case in the lower right corner. From the consequences of the impact - the screen does not work, but the external monitor is quietly picked up.
In Windows (and in Catalyst) there is a snap-in for managing monitors and they define the main screen, but it is not active and is not activated in any way.
Citizens of iron gurus, tell me how the output device sends a signal i.e. how does the PC determine whether it is active or not?

Answer 1

The presence of a monitor is determined by resistance, for old windows ATI drivers, you can even make a special plug of three resistors .