You can rig a little red LED on the door. You will need to dismantle the lock, which is easy, by taking out the two screws. Mount a battery, and micro switch within the cavity. Position the switch so it turns off when the lock is engaged. Drill a small hole to insert the LED so it will be visible from the desired direction. Flashing LEDs are also available, too!

[![enter image description here][1]][2]


  [1]: https://i.sstatic.net/Jzdarm.jpg
  [2]: https://i.sstatic.net/Jzdar.jpg