On my Suburban HW Heater when you turn on the electricity side the switch on the inside of the trailer will light up so you know it's on, you definitely want to make sure it has filled up with water first or you will burn out the electrical element. On the gas side, tanks must be open, you go to the gas switch, mine says heater and switch it on, a small red light next to the switch lights and after the gas ignites on the HW heater it should go out to let you know it's lit, and you could hear it running from the outside. On that gas side that will happen if the water is cold or hot, it's really to know if the Gas side has fired up. There really isn't a way to tell if the water is hot until you open the tap and let the water run, but it does take awhile to heat up 10 or 12 gallons of water. Hope this helps
The is also another switch that will prevent the Hot water heater from work, gas or electric, it's located on the heater, ours is a small black rocker switch that says on/off. Don't let it get wet when you are draining the HW heater, it could short our, don't ask me how I know.
