An old time beekeeper once said "catch a returning bee, without pollen, and squeeze it. You will squeeze out its stomach contents" Never tried it.
When the flow is on and you open up a hive they are usually calm, during a dearth, the bees will bee agitated, and the other bees in your apiary will quickly try to start robbing out that hive. It will get out of hand real quick. Many a time I have tried to remove honey during a dearth and had to stop half way into opening up the second hive.
Feel the weight of half of the hive weight. Leave about 1" overhang on the back of the hives and lift them just enough to feel the weight. When you do this on a regular basis, you will notice they are adding weight real quick, during a good flow.
Jim