Get yourself a stethoscope and remove the round end. You can stick the tubes into the entrances and hear their buzzing.
Last winter I was too scared to do anything to them. This winter, I did open them a week ago or so. It was in the low 30's, no wind and sunshine. I have the Mt. Camp on top, so I took a long stick and stuck it into the upper entrance to feel the sugar. On two hives it felt like there was not much left, so those are the ones I opened. I'm glad I did, too. They had consumed quite a bit of it. I just lifted the lid a bit to peek and slip in a sugar brick. It was really cool to see them!