Fishfinders usually have a "Fish Symbol" feature, which will display a FISH symbol for almost any echo not connected to the bottom or the surface. More specialized units use a detection of a special fingerprint in echosignals. With those you can find living fish and fish that have recently died. With these units also in some cases fish spawn will be shown as a large fish. Some kinds of seafish will be not marked as fish, but you will see an echo.