Watch them from the side.
Full bees approach the entrance with their hind end dangling about 45%
Empty bees are more horizontal.
Similarly robbers exiting a hive are full and can be seen to walk around on the front before they take off or dip significantly as they leave the entrance. Empty bees on a mission fly straight out at speed without any preliminary walkabout.
See if you can get Storch…“At the Hive Entrance” as a download somewhere. It’s fascinating reading.
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