Energy audits are usually performed with thermographic cameras and blower tests, but Alex Wilson of BuildingGreen.com describes what I would call the Yogi Berra Energy Audit: "You can observe a lot by just watching."
The principle is pretty simple: the less insulation in a house attic or at the rafters, the more heat escapes through the roof. That escaping heat melts the snow. As I'm driving into town, if I see that most of the nearly two feet of snow we received is still sitting there and the depth even, I can be pretty sure I'm looking at a well-insulated and tight house.
Alex explains what to look for, how to read the eaves, the ice dams and the surface. And, being a responsible writer, he feels it necessary to conclude "This blog isn't at all meant to suggest that we should skip a "real" energy audit." But it is a good place to start. More at Building Green