When you have this spell prepared (or know this spell and have at least one epic spell slot available, if you are a spontaneous caster), you can sense all spells being cast from as much as 900 feet away from you. You only sense that a spell is being cast; to actually identify the spell, you must make a Spellcraft check as normal, with a -40 penalty on the check if you cannot actually see or hear the spellcaster; plus, you don't know whether a caster you cannot see is friend or foe, nor can you naturally distinguish between spells being cast less than 30 feet apart from each other without actually being able to see the spellcasters (it just looks like a big blur of magic on your magic radar, though you can sort the individual spells out by making a Spellcraft check - DC = 100 + the combined