A few specially enchanted bells can suss out a hidden foe.

Daily   ✦     Arcane
Minor Action      Personal

Requirement: You must have a trap-making kit on your person.

Effect: You set an alarm trap in one unoccupied square adjacent to you. It lasts until the end of your next extended rest. If an invisible creature enters a square within 5 squares of the trap, the trap is destroyed, and the creature loses invisibility and takes a -10 penalty to Stealth checks (save ends both).

Published in Into the Unknown: The Dungeon Survival Handbook, page(s) 20.