If you're not using your water, the tank should stay full. If you want to be sure, give it a good hard thump with the side of your fist. You should be able to hear the water in it; it will sound like a muffled drum. It will also be heavy; 40 gallons of water weighs around 330 pounds

If the tank sounds hollow, or is light and easy to move, then you know it's not full. The thing to do in the is scenario since you don't have a dedicated breaker that you can shut off, is disconnect the line at the top of the tank.

!!!!! If you have to do this, you WILL need to turn off the breaker while until you are done working. !!!!!

On the top of the tank is a metal plate with flexible metal conduit running into it. Once you have the breaker shut off, remove the screw holding this plate down and you will see the wiring. Looking at the wiring coming in from the conduit, there should be two coated wires (most likely one black and one white) and a bare wire. The bare is your ground; leave it be. the other two wires are secured to the heater wires with wiring nuts. Unscrew the nuts to disconnect the wires, and screw them back on to the wires coming in from the conduit.

Since You'll be reconnecting the heater as soon as you can, you don't need to worry about tucking the wires back and replacing the plate, just make sure no bare wire is exposed or in danger of getting wet. You can now turn your breaker back on.