Is it possible to use a spell such as detect magic or
detect evil to detect an invisible foe?
Yes, but not very efficiently. Let’s say a character uses a
detect magic spell; the spell reveals nothing about the
invisible foe unless the character happens to aim the spell
the area containing the invisible foe. If the foe is using an
invisibility spell, a spell-like invisibility power, or a
supernatural invisibility power, the detect magic spell
merely reveals that there is magic somewhere in the area.
The detect magic caster has no idea where the magical
aura is, what sort of creature or object bears the aura, or if
the aura is in motion or not.
One round later, the detect magic user can search for
magical auras again. If the user aims the spell at the
invisible foe again, detect magic spell will reveal the
number of magical auras on the foe and the strength of
the strongest aura. (The user has scanned the same
subject for two consecutive rounds.) The detect magic user
still has no idea exactly where the foe is, what the foe is
like, or whether the foe is moving.
After another round goes by, the detect magic user can
scan for magical auras once again. If the user is lucky
enough to catch the invisible foe for a third time, she will
have scanned the same subject for three consecutive
rounds. The detect magic spell now reveals the strength
and location of each aura. The detect magic user still does
not ioseelo the foe and does not know whether it’s moving
or not. She only knows the strength and locations of
magical auras during her turn in the initiative order. In
this case a ialocationl. is the 5-foot square that contains the
aura. If the creature or item bearing the aura takes up
more than one square, the detect magic user can get some
idea of its size. (See Big and Little Creatures in Combat in
Chapter 8 of the Player’s Handbook.) It is possible for the
detect magic user to attack the location of one of the auras
the spell has revealed. If the spell revealed auras in
different locations, the detect magic user still might not
choose the right location to attack. Even if she does, the
foe has 100% concealment and the attack has a 50%
chance to miss no matter what the attack roll is.
Remember that all of the foregoing depends on the
detect magic spell user scanning the invisible foe for three
consecutive rounds. If the detect magic user guesses
wrong about where to scan even once, she’ll have to start
the process of zeroing in on the invisible foe all over
again.