If and when I am the listing agent for the seller, I usually ask them to give me their electric and gas bill
expenses - which most of them willingly do, if not on day one, they will provide the information at one point or as soon as an interested buyer asks the question...
If you are the buyer and I were your buyers agent, i.e. working on your behalf as the buyer, I would ask the
Listing agent to get that information from the sellers as soon as possible so that my buyers can have an app. idea.
Now having all that said, there is a huge difference in how people live and use their home and consequently how they heat and cool their homes. Some use a really good thermostat and keep the house in the winter at a relatively cool temp for the night and others do not, so whatever that particular home owner pays is just an app. guideline to you.
Once you move in, you possibly can make changes, you can put in better insulation in the attic, or may be consider new windows etc. and that way improve your monthly utility costs...
But what the current owners paid as an average throughout a year is a good indication what it will cost you.
And then remember there are some basics that make one house more or less expensive to heat or cool
one floor with finished basement, 2 or 3 or 4 story homes, split levels etc.
I hope I helped a little with my input, work with a buyers agent who works on your behalf and he/she will help you get the questions you have answered.....
Take care and good luck