The caster dangles a weight from a string (a plumb bob) held in their mouth. The caster performs gestures of balance and discord while humming The Scales of Balance. How the plumb bob responds by twisting or swaying indicates the degree of Evil or Good and the degree of Lawful or Chaotic attitudes of the targets. Each target requires only a couple of seconds to analyze the results, so the spell can Detect Alignment of 3 targets per round in combat. Otherwise, for the duration, you know the Alignment of all creatures and persons within 30 feet of you, as well as where the creature or person is located. Similarly, you know if there is a place or object within 30 feet of you that has been magically attuned to an alignment.
The spell can penetrate physical barriers, but it is blocked by magical elemental effects (wall of flame, etc) or the presence of Greater Beings (powerful celestials, demons, devils, fey, undead, etc) whose alignment is both obvious and not the same as the caster.
* - (Any string with a weight tied to one end; a plumb bob.)