A know alignment spell enables the priest to exactly 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 a full round. If the creature rolls a successful saving throw vs. spell, the caster learns nothing about that particular creature from the casting. Certain magical devices negate the power of the know alignment spell.
The reverse, undetectable alignment, conceals the alignment of an object or creature for 24 hours.
Notes: Common for clerics and druids (PHB).