The cosmology is closer to that of 4th Ed. The Feywild and Shadowfel each ‘mirror’ the ‘normal’ world (known to Elves and most extraplanar beings as ‘the Fadelands’). Though it is fairly rare to find a ‘crossover’ point either one (particularly the closer you are to cities and masses of population),there is a common thread among fairy tales told to children: if you somehow wind up in the ‘green lands’ or the ‘shadow lands’ (they have many names),stop immediately and backtrack until you get back to where you belong!
Interaction with the Feywild and Shadowfel by PCs is more common (and at lower level) than is usual in 3.5 Ed, whereas the Astral Sea and Elemental Chaos are virtually inaccessible by any mortals (save for brief contact with the Astral when using teleportation-related magic). Elysium and Hell are even more distant, though their respective deities interact with mortals in the form of dreams (and nightmares, respectively). Certain races are sensitive to specific planar boundaries. Having somebody in the party who can tell immediately that you’ve crossed into the Feywild/Shadowfel is certainly not essential, but it can be handy!