By the specifics of the rules, a monster knows the details of any conditions on them. The thing to keep in mind is that different marks work differently.
A paladin, for example, subjects someone to their Divine Sanction. This is one complete effect - the monster knows that they are marked, and will get zapped the first time each round they violate the mark.
A warden or fighter, meanwhile, simply marks an enemy. All the monster knows is that they will take a -2 penalty to hit if they make an attack that doesn't include the fighter. Now, many will likely be smart enough to know that fighters often punish those who violate their mark, and they may suspect something will happen. But they don't know for sure until they see it in action.
For me, this is fine - more magical marks like Divine Challenge and a swordmage's Aegis are more obvious how they will act upon the victim, while with Fighters and Wardens, all that is obvious is that they are in your face distracting you.
That said, I don't think there would be any game balance issues with declaring that monsters always know the penalties for violating a mark, or that they never know until they see the effect happen. Just be prepared to make the same hold true for players, and all should be fine.