The deceptions were manifold. These ranged from the building of artificial airfields complete with papier-mache aircraft in East Anglia, to radio traffic deception by a specially briefed outfit which drove around the south-coast of England simulating an army manouevring, to the broadcasting of misleading messages from secret agents who had effectively been 'turned' by the Double Cross System, such as /Garbo.
The Allies were able to judge fairly easily the effectiveness of these strategies since they had cracked the German code-system fairly early in the war, and were thus able to decrypt the German high command's responses to their purported overtures.