I don't think anyone mentioned this but in the PHB or DMG ( I don't have them in front of me so I can't site which book or page but it is in there) it says that foes are aware of powers that affect them, so when a paladin challenges them they understand that if the don't attack the paladin they will take damage.
Yes, this is in the RAW.
But I don't feel it applies to every possible action that might occur in combat.
If a power is used against a creature, I feel that the creature knows something happened. Even unintelligent creatures have awareness, and instinct, and a desire to survive. Usually that is enough to realize something just happened, in most cases, recognize that certain decisions would now be bad and certain decisions might be good.
Non-intelligent monsters like slimes and certain undead or constructs may lack the decision making process to choose actions that promote survival over actions that lead to pain or imminent destruction, but everything else has at least rudimentary instinct to preserve itself.
But not all things are powers. Not all things affect the foes in ways foes can detect them.
For example, that fighter ability to take a free shot at a marked foe. Sure, the foe knows he is marked. Sure the foe knows that the fighter is now the easiest target to hit and damage. But the foe doesn't necessarily know that if he tries to move away the fighter will take a free shot at him.
As a general rule, for me as a DM, I assume that powers have tangible effects that can be seen, heard, or at least felt, by just about anything with any kind of sensory perception, and given that, the creature can then take steps to maximize its own survival, assuming it has even a rudimentary intellect.
But, as a general rule, I assume that feats usually don't announce their presence to a foe. There is usually no way to know what that foe is going to do with a feat, unless you've seen him use it before and assume he can do it again.
Then there is a little gray area where some powers, or some magic items, have effects that seem like they might not be easy to detect or recognize. These I make a case by case ruling.
