A know alignment spell enables the wizard to read the aura of a creature or an aligned object (unaligned objects reveal nothing).
The caster must remain stationary and concentrate on the subject for two full rounds. A creature is allowed a saving throw vs. spell and, if successful, the caster learns nothing about that particular creature from the casting. If the caster concentrates on a creature or object for only one round, he can learn only its alignment with respect to law and chaos.
Certain magical devices negate the know alignment spell.
The reverse, undetectable alignment, conceals the alignment of an object or creature for 24 hours - even from a know alignment spell.