As Maria said, you can call the utility company. They will ask you a series of questions, about your family size, if any one is planning to be home during the day, what appliances you will planning to use on a daily basic, etc. and they will then calculate the average costs for your family. I have found this to be very helpful for my clients.
Yes your usage will not be the same as the current homeowner however you can call the utility companies directly for this information. You will need to provide the address of the property. With utility deregulation in effect PSEG is not the only provider in the area for gas and electric service they still do the billing for nearly all of the available providers. 800-436-PSEG (7734)