stephan2307:
I see it that there would be a small Arduino in each emergency light. Mains to the unit would go through a relay controlled by the Arduino.
This latter sounds quite reasonable. Note that the lights will be connected to the normally closed relay contacts, so they will be operational when the relay is not actuated, it will "fail safe".
stephan2307:
How can I check if LED lights actually work? Just check the current it draws during the test?
No! You need to check that the LED lights actually work. This means that you check that they produce light as has been explained already! 
In other words, you use light sensors.
Note now that this means there is absolutely no modification to the emergency light units themselves, they are not touched in any way whatsoever and the monitor is not even nearby.
In my building, the emergency lights are on the common lighting circuits, one for each floor. The test is to switch off that light circuit; it becomes dark except for the emergency lights. So if you use light sensors and particularly if you arrange to perform the test at night ("out of hours") there is no confusion as to whether the light comes from the emergency lights or the common lights.