Detect if a microwave is running
Anything you wire up to each power outlet won't be cheap since someone has to wire it up to each power outlet.

Presumably there is some display on each microwave that indicates if it is still working, or the cooking time has expired.  Install a video camera in the microwave room with a monitor in the dining area.  Then people can look over and see if their microwave is done yet.  If you have enough resolution, they might even be able to see how long until it is done.  For more resolution, divide up the bank of microwaves between multiple cameras.  That will still be a lot cheaper than someone designing and wiring up some kind of monitor circuit for each unit.