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.