Go outside, open water heater door. Lower left corner, behind a pipe, or burner tube, there will be a black rocker switch, If the unit is indeed gas/electric. Switch may have a locking pin, looks like a hair pin. That insures you don't turn on electric until the heater has water in it, else the element will burn out in less then a minute. If there is no switch, your heater is gas only. Also the model number will have an E if it is electric.
__________________
|