On a Dometic, the only way is to either test for hot on the cover or buy a clamp on AMP meter and test for the amp draw at the wires of the Heat Element. Doug
ANOTHER WAY----- Buy a CHEAP 120 non digital clock and wire it thru one of the 120 Heat element wires. When on 120, the second hand will be turning. I guess you could do the same with a 120 digital, it would then flash when 120 was supplied.
SORRY, Another easier way is to but a standard 120 volt small round LED light and install in the RV by the refer. Wire it thru one of the 120 element wires and when on 120 that LED light would be ON. Doug
Like this, Just need one tho. BUT will only show that the element HAS 120 volts, not if it is Heating
https://www.amazon.com/YC-9WRT-1R-120-10-INDICATOR-MINIATURE-PILOT-LIGHT/dp/B01E2UITTC/ref=sr_1_115?dchild=1&keywords=miniature+light+bulb+120+volt&qid=1601238731&sr=8-115
