During the previous weeks, I explained how to turn the ScreenSaver on/off. But how do you know if the screensaver is actived?

Try the following function:

function ScreenSaverActive : boolean;

begin
  SystemParametersInfo(SPI_GETSCREENSAVEACTIVE, 0, @Result, 0);
end;

This function returns TRUE if the screensaver is active, or FALSE, otherwise.