to test your CGA monitor,
1. use a pc with DOS as yopur OS
2. Make a VGA breakout cable (click in the button for diagram)
3. setup ArcadeOS or Advmame (other DOS FE that works with Arcade Monitor) boot straight to FE
To test the monitor in a not to safe way, just use a Breakout cable, connect to the monitor turn pc on and if you see DOUBLE IMAGE on screen, monitor is GOOD.
Just turn off right away after seeiung image. This should not hurt the monitor at all (just for that few seconds)