The size of the rise will disclose the size of the insect the trout is feeding on. At least at the distal ends of the insects scale of sizes. We will infer whether a trout is feeding on tiny midges or on large mayflies. Conversely, it won't be easy to distinguish a rise on mayflies from one on a mid size other type of bug. Unless we see the bug of course, which is not always easy by standing on the riverbank.