The problem: A thin metal or plastic plate isn't enough to prevent air from getting through what's basically a big hole in the wall.
How to spot it: Remove the plate and cover the opening with a ply of tissue affixed to the wall at the top with painter's tape, like a curtain. If the tissue billows, you've got a leak.
How to stop it: Insulate the opening with a precut foam gasket, about 10 cents each at home centers. Just fit the gasket over the opening and replace the cover. For extra protection for outlets when they aren't in use, insert plastic child-safety plugs.
The payoff: Two percent off heating and cooling costs.