You are smart buyers to try to factor ALL costs of owning a home not just the mortgage! The easiest way is to get your agent to request copies of the last 6 to 12 months of utility bills from the sellers agent. That said, please keep in mind they could vary significantly to what YOUR usage would be. The bills are often more specific to the occupants habits than the house. If you are a couple with no children your water bills and most likely electric bills will be a lot less than someone with five kids who take long showers, generate loads of wash, are in and out of the air conditioning all day etc.
Most sellers are prepared to provide this information if you are seriously interested in their home.