The PCs generally know where all enemies are and vice versa (except those who are Hidden). They also know the effects of all zones and other buffs that are in effect. This is part of the "all the cards are on the table" philosophy of 4E. They may or may not know the monsters powers, HP's, vulnerabilities, etc. depending on whether they make their Knowledge rolls (but they'll figure them out soon enough).
My question is, do the PCs know whether they have Line Of Sight or Line Of Effect to the monsters? My guess is that they obviously know whether they have line of sight, since a blocked line of sight is equivalent to Partial Concealment (-2 to attack) or Total Concealment/Invisible (-5 to attack).
But I'm not certain whether the PCs know know for sure whether they have line of effect (partial cover for -2, superior cover for -5, or no line of effect meaning the attack fails). Suppose there's a glass wall, or wall of force there -- do they know about it automatically?