I often use a cheap stethoscope to monitor my honey bees in the winter when they’re still clustering below the top bars and out of sight. It’s the least disruptive method I have for checking on the bees.
It took some practice, but I can tell how deep and how large the cluster is by listening through the hive with the stethoscope. Most of the time, though, I’m just checking that the bees are still alive. That’s usually good enough for me.
Sticking my ear against the hive works too, but it’s not as dignified as walking around with a stethoscope.
FEBRUARY 20, 2016: I have to say I continue to be impressed with the $7 stethoscope I bought on Amazon. I listened again to my bees today and could hear a lively buzz of bees in every hive. It takes a bit of imagination to interpret how the bees are doing from the often distant-sounding hum heard through the stethoscope, but at least I can tell they’re still hanging in there.
April 2019 Postscript: I still have my stethoscope, but I don’t use it often anymore. I just bend a knee into the dirt and stick one ear to the side of the hive. I can hear the bees better through my ear.