My clue is when the little ladies are flying from my apiary blind (a wall of shade-cloth 6' high, along all 3 sides and over the top with a 1 foot wide opening just above the hive entrances) in large numbers, a cloud of bees above the blind opening. At the same time vigorous activity, coming and going, is evident at nearly every colonies entrance. Many of those returning are carrying pollen. More difficult to determine if those with a distended abdomen are carrying nectar, stolen honey, sugar water, soda pop, honeydew, or water. Another clue was when I scraped some burr comb containing honey onto a board immediately adjacent to the apiary. About 6 feet away and the bees didn't even bother with it. Two days later I rinsed it off with water from a hose.