I am casting around for images to enter into a competition and note the rules state should be taken in the last 5 years. How would anyone know? Of course a digital file has the EXIF data but if I send in a scan of 35mm or 6x7 reversal, they might ask at some stage to see the original film. There are are markings other than the film type but appear to be some sort of bar code. Of course that only indicates when film was manufactured, not exposed, but if the film was manufactured in 1998 with an expiry date of 2002 (say) might be a stretch to convince anyone it was exposed in 2006.