If you have excess algae you're overfeeding your fish. If you don't, then you're doing it right. However, if your corals appear to be starving then you're not feeding enough.
The way I see it, overfeed is relative to the capacity of your skimmer and nutrient export, not necessarily how much your fish can eat, because we want some nutrient in the water column to sustain our corals.
Since I currently have a large tank with few fish, I feed a lot, not because my fish really need that much food, but because I need those nutrients in my water to sustain my corals.
That being said, I don't just dump tons of food in the tank, I feed often and enough food that my fish eat as much as they can, not more than they can...