If you need such a gadget, you are doing it wrong. Learn the fundamentals: Line up the sights correctly with focus on the front sight, press the trigger without disturbing the sight picture, and follow through. If you do that, you know exactly where your shot went, including during dry practice, because you know what the sight picture was before, during and after firing the shot.
If you do not know exactly where your shot went, then you are not lining the sights up correctly with focus on the front sight and/or you do not follow through. If you do know exactly where your shot went but you are all over the place, then you are not pressing the trigger without disturbing the sight picture.