... An indicator panel with 4 neons, for the elements, l1/l2 and the overload?
Green (lens) across the input voltage
Red across the over temp sensor
Yellow across each element.
For a couple of bucks you would instantly know the electrical status of the heater and with a simple flow chart you could diagnose the problem without ever opening a cover or getting out your meter.
I suppose you also need one between the thermostats if you really want to cover all bases.
