I used a stethoscope for years to locate clusters. Worked very well even in temps way below zero, which was when I tended to check. Pretty cool to hear the bees humming in the 'dead' of winter and while the trees are 'cracking' due to extreme cold.
This year (first time) I placed 'rigid foam insulation shells' around all of my hives, so the stethoscope will (likely ?) go back into the medical drawer.
I did pick up an IR Gun for under $20.00 a few years back but again, I'm not sure it'll give me the kind of reading needed w/ the foam shells so we shall see.
Ahhhhh, the bee-waiting game