If you have an Atwood heater, lower the door on your heater, the model number will be on a sticker on the right side of the interior frame:
G = Gas (propane)
C = Combo (gas + electric)
H = Heat Exchange (engine)
6 = 6 Gallon (or 10 for 10 Gallon)
AA = Heating Element
8 = Revision
E = Electronic Ignition
P = Pilot light
For example, my heater's model number is a G6A-8E (gas, 6 gallon, Electronic Ignition).
If it were a 6 gallon gas/electric, it would be a GC6... something.
Another way to tell is to look on the back of the heater. If there is an electric element, you will see it there. This is my heater, and there is no electric element:
If there is an electric element, there will be a black box on the backside of the tank like the one shown below:
Of course, this is a 6 gallon tank, a 10 gallon tank will look a bit different but have a similar box/lack of box.