Is there a "flow"?

[K-R-Farms]

So that's the question some folks ask.

but, how do you tell if there is a nectar flow?

[Scadsobees]

You can see the honey gauge on the side of the hive increase rapidly. :-D

Much of it comes from knowing what is blooming when. 
Then you can watch the weather, lot of rain + good weather = good flow, months of hot dry weather = no flow, weeks of cool rainy weather = no flow.

Finally, watching the bees...if you see lots of new white wax in the hive, they are filling up supers and frames rapidly, those are two good signs.