You're giving me a headache
I think it would just be silly to try to establish a piece of information that's clearly been generated by the game as fake (i.e. a scribe note; an assassination attempt notification; tax returns; etc). The game says hero Bob has been killed in battle. Your character says that the scribe who issued the notification was drunk and Bob is well and enjoying his breakfast. It doesn't change the fact that Bob's really lying in a pool of his own blood and soon to be heading to the family tomb. The fake version of events won't be accepted without equally convincing proof, which is impossible to obtain.
Characters accept this because they exist in a shared reality where the scribes sending out that sort of information never get it wrong. Questioning it would never occur to them because their reality would break to consider such a thing. Information generated by game mechanics are the laws of nature of their existance, thereby trumping everything else. The comparison in the real world is like someone saying to you that gravity has stopped working. So you throw an apple in the air and it comes down on your head. The laws of nature clearly don't support the fake argument.
Of course, letters can be faked because they're subjective. So a character might always be suspicious of something passed to them through a third party.
