With many houses in Northern Wisconsin being heated with propane and fuel oil, a well insulated house can save you money every month of the year. There are four main things to look at; insulation type, air tightness, windows, and construction method.
There are basically four types of insulation used in homes. Fiberglass, cellulose and open cell spray foam all have different things to look for. If fiberglass insulation is installed in a sloppy manner (very common) it does not do a lot of good. If cellulose has not been dense packed, it could settle and leave a void at the top of the wall cavity. Open cell spray foam can soak up water if used improperly. Closed cell spray foam is pretty bulletproof, although when a void isn't completely filled, convection currents in the walls can lead to drafts, but not as severe as you would get with fiberglass.
You will sometimes hear people say that modern homes are built too tight and a house needs to breathe. Don't listen to people who say that. The building science industry has proven over and over that a tightly sealed home is a more comfortable home that costs less to heat and cool. Even at extreme levels of air tightness a house still "breathes" plenty. A tightly sealed house will have caulk on all the wood to wood connections, a taped air barrier on the outside and many other details that an unsealed house won't have. While these aren't things that can be verified without a blower door test, you can and should ask questions about how the house was built.
Windows are something that are a bigger factor with older houses. In a lot of older homes there will be single pane glass windows that are ill-fitting. Poor construction techniques often lead to drafty windows, and drafts are simply unfomforable to live with. Single pane glass is just a poor insulator. Ideally a house will have double paned windows with a gas injected between the panes for better insulation values. There is a movement towards triple pane windows but the only time they really make a lot of sense from a payback perspective is if the rest of the house is superinsulated (very rare) and even then the value is debatable if natural gas is your primary fuel source.
The final insulation related aspect to look at is construction method. The best way to increase the R value of a wall is to break the thermal bridge created by wood studs. Wood has a r-value of about 1 per inch. Wood studs often make up 25-35% of the total wall area of a house. This means if there is no break between the outside of the stud and where it meets the drywall, 25-35% of your wall will have an R value of about 5.5! Thes best way to create a thermal break is with rigid foam insulation on the outside of your house. Other effective methods are double wall construction and staggered stud construction. The less thermal bridging a home has, the more comfortable it will be and the cheaper it will be to heat and cool.