The only time I feed is to prevent a colony from starving. A good sign of nectar coming in is when you see a lot of glistening on brood frames*, also look to see if the honey arc above the brood has honey in it. If I see empty honey arcs with no other honey in the hive, I’ll feed the colony with honey in an upside down jar, stickies if available of move a frame of honey from another colony.
*Foraging bees will offload unripe honey straight on top of brood, then I’m guessing at the end of the day, or even during the day the unripe honey gets moved away to somewhere more permanent.
**Most people call it nectar, however, depending how far away the nectar is gathered, the bees start the de-watering process & mixing with enzymes on the return trip, which turns it into very unripe honey.