Considering that you watch your girls every day, you should be able to recognize the change in their behavior. When the flow is on, the bees enter and leave the hive as if in a frenzy. They want to get to the source of the nectar as fast as they can and return and unload in a zip. No more leisurely coming and going. Often, when a flow is in full swing and the girls have entered into the swing of things, you will see them bumping into each other as they enter and leave. It's really quite a sight.
During the nights, you'll hear a happy hummm near the entrance, as the bees vibrate their wings to speed up the evaporation of the water from the nectar so as to concentrate it and make room available for more inside the cells. Many bees will be standing at the entrance during this process and you can often smell the concentrated fragrance (REAL perfume) of the flowers they've been working.