There is no simple visual marker to tell you. The yellow wedge on the boats you aim at is all you get. As I said it only goes red, and that ONLY happens if the target is in a "good" shooting position (eg yellow/green NOT orange). In anycase the lock button lighting up or not is nothing to worry about. I never even look at it anymore anyway. Easy aim is a case of simply point the crosshairs where you want to shoot, open the tube and hit the big red fire button. Job done
*EDIT* Ah well I didnt think of the auto reload being off. Nice work there Peabody.