I can tell when a flow is on because my bees stop taking sugar. If its a light flow they just slow down on the sugar.
That is true. But if you have flow, why to feed with sugar?
It restrict the brooding area and mi sugar to honey.
If hive has good flow, and you put open honey outside, bees go to flowers and not suck the honey.
But when willow ceases, bees start to rob other sources than flowers.
Last Spring I had a hive among big blooming willows. Hive was almost starving when I brought it the place.
Couple days later I went to give sugar syrup. When I opened the hive, they had brought in 2 days more willow honey than I had syrup with me.
But of course, if it becomes rainy week, they eate it all in very short time.