Combination of putting in measured amounts of feed (I do by the scoop), checking the amount of feed in feeder at roosting time, and crop distention. To start, I track how much feed is applied in the morning to empty feeders. In the evening as shutting pens for the night I make a quick glance at feeders. If a feeder still has feed in it, then I reduce allotment the following morning. If feeder empty, then I first visually check crop distention of lower ranking birds if known. If in question then I walk over and tough the crop area to see how well it is distended. I often had to check birds late at night which was ideal time to verify at least some crop contents half way through the non-feeding period of the night. With adult birds feed intake will vary somewhat with weather, especially temperature. Juveniles require closer monitoring and behavior in the morning can be informative; learn their language indicating stronger than usual interest in feeding.
You can also do this with free-range chickens to optimize foraging.
