Today's a good day to check - if you've had a snow fall. Just look at the roof. If there's loads of snow on it, it suggests a well insulated roof. If the snow has melted away, then it suggests poor insulation. Or am I talking rubbish?
Here's a photo of a couple of rooves on a pair of semi-detached houses, which I took today. The one on the left seems clearly better insulated. (Or maybe one householder has the central heating turned up higher than the other).