A document that is forged must have a footnote that reads:
OOG: Forgery, + X Profs, Card ###
A document that is signed with the seal of a Baron, Cardinal, or other (higher) ranking official must be signed:
OOG: Authentic, Character Name, Card ###
In both cases, the term "Card ###" is replaced by a Marshal's, Storyteller's, or Director's name in the case of an NPC.
Note that the use of a seal is not mandated by a character, if they wish to write an unofficial document (such as a personal journal) - and such documents can still be forged.