Not quite true, at least by a strict reading.
You know the exact specifics of the condition itself. So you know how Horrid Link affects you. You know how a Paladin's Divine Challenge affects you. You know how a Swordmage's Aegis affects you.
If you are Marked by a Fighter, though... you know that you've been Marked, and the basics of the Marked condition. You don't know about the Fighter's ability to take a free attack on you if you violate his Mark, however.
Essentially, if the details are contained within the power or condition itself, they are obvious to you. But if the enemy simply has access to other powers that interact with that condition, you don't know about them.
If a Warlock curses you, for example, you know that he will probably do extra damage to you with his attacks. But you won't necessarily know that he has a power, Cursegrind, that lets him attack just his Cursed targets, for example - or if he has a power that will have additional effects if he uses it while you are Cursed.