No rule of thumb that I'm aware of. Just a few warning signs. Since it's an electric/propane water heater, couple of warning signs. I went to the electric mode, and all looked well (the switch in the water heater compartment was "on" and illuminated.) But no hot water. Discovered a GFI outlet on the same circuit was tripped. Then pulled off the cover to the two thermostats and one of the wires to a spade connector was "burned" brown. And finally, I checked the resistance between the terminals on the heating element. I think it was around 20 Ohms. A little high, might be an indication of a bad element-or more resistance because the element is damaged.
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW