A fish feeding on the surface will have two distinct looks. The first is when it eats a bug sitting on top of the water surface. It will stick only its snout out of the water. You can often see a bug float into its range and anticipate the eat.
When most fly fishermen think of flies for spring they usually consider either large streamers or nymphs fished slow and deep. Dry fly fishing is usually a second thought and normally only if the angler sees a fish rise to a bug or, as often happens, hits their strike indicator.