I'd start poking around outside and down in the basement. Is there a gas meter? Does the meter change value/spin/whatever (sorry, I don't have NG at my house, can't remember exactly how it looks from childhood home)? What kind of heat do you have: forced air, baseboard registers, radiators? What about your water heater (electric, gas, oil)?
It's possible that your base gas bill includes a small # of base therms, and you are not exceeding that. If your stove is gas, and has flame, you must have some kind of gas hookup. It could be LP, that's what we have for our stove.