Right - even a basic melee attack is a power, and so when the fighter hits the enemy with the basic melee attack, the enemy takes damage and acquires the marked condition. He knows he has acquired the marked condition.
As to whether the enemy knows he'll be subject to an attack if he shifts or attacks someone other than the fighter, I'm not so sure. The ability to attack a marked target is a class feature of the fighter, not a result of the power that imposed the mark.
If I have a personal range power that says "You deal +2d6 damage when you attack a creature that has the weakened condition", and my friend the Warlock hits an enemy with a power that imposes the weakened condition, what does the enemy know?
"You are weakened"?
or
"You are weakened, and that guy over there has a power that will grant him +2d6 damage because of it"?
I'd contend the enemy only knows the result of the power (he's weakened) and not indirect consequences (another power exists that can take advantage of the condition). Similarly, I'd contend the enemy struck by the fighter knows the result of the attack (he's marked), but not indirect consequences resulting from that condition (he's subject to attack if he shifts).
-Hyp.