Walls are important to consider in an insulation evaluation because percentage-wise, they are a large portion of the building envelope. However, because of the difficulty and expense involved, insulating walls in an existing home is hard to cost-justify.
You can check to see if your walls are insulated by removing an electrical outlet cover and looking into the wall cavity. Turn off the circuit breaker first for safety, and if you need to use something to probe around, a plastic knitting needle works well. If there isn't any insulation, you might consider getting an estimate on blowing loose-fill or foam insulation into the walls.
Blowing insulation into frame walls involves drilling holes near the top of each stud cavity and another lower down on the wall if the stud cavity is obstructed by a fire-wall or window. Once pumped full of insulation, the holes are plugged and covered. It is a fairly expensive and involved measure, but one of the few cost-effective ways of adding insulation to existing walls. An alternative is applying rigid insulation to the outside of the home and covering it with siding.