electric / propane water heater question

There is no way to verify what is happening on 120 on the Water Heater. If you leave it on BOTH LP and 120, then the LP will light when calling for heat as well as the 120 element will be heating also. The only reason to have BOTH LP and 120 On at the same time is faster recovery when using Hot water. When you go to sleep turn the LP side OFF and let the 120 side do its job. If you want faster recovery in the morning when showering and such, flip the LP side back ON. The once you have stopped the high volume usage of hot water, turn the LP side back off.
HOW to know if the 120 is working? Leave the LP off and let it run on 120 overnight. If you have HOT water in the morning, the 120 side is working. Doug