Sunder's suggestion of introducing information and watching for their response is good, though the Adversary will not respond if they're smart and they're watching you for something more important.
(What was that Brit town sacrificed so the Germans wouldn't know the codes were broken? Starts with "C"...)
Coventry. Story goes Churchill had info from Bletchley Enigma intercepts that Coventry was next to be bombed, but if he evacuated it, the Germans would have suspected that Enigma was cracked. I've also heard references that this was an urban legend, not sure.
However, they did sink quite a lot of subs based on triangulation + Enigma decrypts by fortuitously sending an airplane in the area they knew the subs would be before hunting them down - as cover to say that the subs were spotted... This way they could sink the subs and still let the Germans think that Enigma was safe.
