Look at the switch to turn on the propane heat in you water heater (normally in bathroom) it has a small flame and red light that illuminates briefly when turned on then goes out when water heater is lit. There should be another switch next to it with a lightening bolt. That is for the electric element. If there is not one it was not part of the original installation. Look at the top front of your water heater where the piping is. There should be 4 wires for duel heat but only 3 for just propane. also on the bottom front left there should be a electric junction box with 3 12 gauge wires coming out (white, black, green).
Dave and Felicia