I have a nectar flow

I checked all my hives today. My package bees that I put in on my May 7th delivery, 6 were on drawn comb and needed a second box. They were very crowded and probably needed a box a week ago. I have three packages that I started on foundation. They are probably a week behind the hives with drawn comb.
 My overwintered colonies are packed with bees. I had no swarm cells. All of my overwintered colonies got two supers put on top of them. They did need them as they have been bringing in nectar.
How do you know there is a nectar flow?

My deep boxes have drawn comb and the comb is on the dark side. Notice the white comb attached to the darker burr comb.

The dark burr comb has new white comb. Whenever the bees add comb to anywhere in the hive, it will be white in color during a nectar flow. So I knew at a glance that this hive was bringing in nectar recently.