The Allies were able to judge how well Fortitude worked because of Ultra, the signals intelligence that was obtained by breaking German codes and ciphers. On 1 June, a decrypted transmission by Hiroshi Ōshima, the Japanese ambassador, to his government recounted a recent conversation with Hitler and confirmed the effectiveness of Fortitude. When asked for Hitler's thoughts on the Allied battle plan, he had said, "I think that diversionary actions will take place in a number of places – against Norway, Denmark, the southern part of western France, and the French Mediterranean coast". He added that he expected the Allies would then attack in force across the Strait of Dover.
The deception was also assisted by very high German assessments of Allied capabilities. In an appreciation of 8 May von Rundstedt said:
Observed tonnage of landing shipping could be taken as sufficient for 12 or 13 divisions (less heavy equipment and rear elements) for fairly short sea routes. In all (estimating the capacity of the other English ports not so far covered by visual and photo recce) probable employment of at least 20 and probably more divisions in first wave must be expected. To these must be added strong air-landing forces.
During the course of Fortitude, the almost-complete lack of German aerial reconnaissance, together with the absence of uncontrolled German agents in Britain, came to make physical deception almost irrelevant. The unreliability of "diplomatic leaks" resulted in their discontinuance. Most deception in the south was carried out by means of false wireless traffic and through German double agents. However, those methods had significantly less impact for Fortitude North. In his 2000 book, Fortitude: The D-Day Deception Campaign, Roger Fleetwood-Hesketh, a member of Ops (B), concluded that "no evidence has so far been found to show that wireless deception or visual misdirection made any contribution". It is thought that the Germans were not actually monitoring the radio traffic that was being simulated.