When fishing, watch the hopperr closely. A take to the sunken fly can be subtle. Sometimes the hopper just stops; sometimes it seems to dart upcurrent a bit or move unnaturally sideways. Often these are indications that a fish has taken the sunken fly. Sometimes the fish will see the hopper and move up for a look, even flashing quite near the surface. Such fish may nail the trailing fly, so be ready. If you're unsure in any of these instances, raise the rod tip until the line is tight. If a fish has taken the trailing fly, you're on. If not, the rig isn't pulled totally out of the water, and you can continue to fish out the drift.