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.
I think this is an excellent breakdown of the issues here.
Disbelieving information given to you by the game
that is never wrong (note: some information given to you by the game has a chance of being wrong: for an obvious example, see the precise details of CS in scout reports) is foolishness, and its logical extension is that your character cannot believe
anything that happens around him, because it's all reported by messages. Nor can he believe that any of the letters sent to him are real.
So...yeah, that's stupid, and leads to pointless frustration when you try to interact with people who are playing the game in a sane manner.
If you're having trouble with the disconnect between what real people can believe and what BM characters can believe, remember this: Real people can meet in person and hand each other gold coins.
