keithgrimes,
By 'simple observation' is the easy answer.
The combs were loaded with a suspicious watery liquid which unfortunately ran out when I inspected one frame on one colony. The bees were later seen scavenging under that hive for the drips which went through the OMF. I suppose they could have filled the frames with river water, but I somehow doubt it. So, for a very warm day I was confident it was nectar.
Similarly, the home garden bees had ceased collecting water in the warmer weather (and the very weak sugar solution in the Dartington remained untouched for several days). They have used some these last two days (I looked, as I need not disturb the frames to do that) and there are large numbers of bees back on the water sources (just outside the house) again.
They are certainly not storing it as honey in a super yet but I did put a drawn-comb super on one hive last week. It may get used as a second brood, but I shall not worry about that.
Regards, RAB