They couldn't really. It was an open secret that the western Allies were preparing to invade, the goal of Allied deception operations was to obscure the intended landing sites. That said, Allied air cover flew constant patrols over Britain to intercept German reconnaissance aircraft to prevent them from photographing the shipping build up and the marshaling areas. Those air patrols would on occasion allow German aircraft to slip through at times and places of the Ally's choosing, usually to allow them to photograph dummy camps that suggested an invasion at the Pas de Calais. German intelligence officers were constantly complaining about the lack of adequate aerial reconnaissance.

Source: Fortitude: The D-Day Deception Campaign by Roger Hesketh