Sense Heretic

Holding the target item aloft, you quietly chant the last few words to cast the spell. The item immediately flares a soft blue color, which fades almost immediately.

(Spell Compendium, p. 182)

Originally posted on D&D tools

Divination
Level: Paladin 1,
Components: V, S, DF,
Casting Time: 1 standard action
Range: Touch
Target: Object touched
Duration: 10 minutes/level (D)
Saving Throw: None
Spell Resistance: No

This spell is usually cast on a weapon or a holy symbol. If an evil creature that has the ability to cast divine spells comes within 100 feet of the item, the item begins to glow with a faint blue radiance. You cannot tell where, or in which direction, the detected creature is, only that such a creature is within range. The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.