The listing agent for the property should be able to get that information. However, that is still subjective to the usage of the individual living within the property, as your agent would be one to tell you. If they use CFLs and are willing to deal with 80f heat, it will be different if you like dimmer switches on Incandescent and relaxing in 72F living rooms.
There are calculators that you can use provided by most utility companies for square footage and what kind of appliances you tend to use.
If you are asking based on Natural Gas, as I am sure you notice from heating fuel, gasoline, and diesel prices, that is NOT a fixed price and is prone to being shifted up and down.
Again, your best bet is to ask your associate realtor or the listing agent for the property, as they can easily get that information from the owner.