Isn't the obvious way to handle this to include an undeveloped (latent image) photograph of some obscure object, person, or place on the film rather than just a blank film ? ? You could then develop it and check for light damage and evidence of lack of authenticity. I suspect there are tricks involving calibrated exposures of objects with known optical power ratios (a kind of hidden grey scale strip) or even holograms superimposed on normal looking photographs of scenes that might be rather hard to easily duplicate by developing the latent image and making either an optical or contact print of it on a similar medium.

The hologram trick is very interesting; could cause a lot of problems for the adversary. Now the question remains, how to make a hologram within the resources of a common person, to make the system suitable for wide use, not only for a handful of high-tech geeks with closets full of cutting-edge gears. Also, how to make sure the image got properly exposed, so it couldn't happen that a mistake of the sender couldn't result in a false alarm. (Maybe to develop part (half, stripe...) of the image and then check under the red light, before using?) The issue starts to look more complicated than it seemed on the first glance. We have a resourceful adversary, who will quickly learn the tricks. We need a low-tech technology that will be highly resistant against undetected tampering by the adversary. Does anyone know if this wasn't already being solved during the Wars, or the Cold War? I am pretty sure many embassies had problems with adversaries going through their diplomatic mail.