Divination: Paper Umbrellas (Perception) No default
It's best to describe this reliable method of identifying demons as a skill, although it's one that's not only limited to celestials only, but also requires a specific "material component" (to use the technical term) in order to even work. To operate, the celestial must first drink an alcoholic beverage that comes with a small paper umbrella - the drink must be one that humanity legitimately associates with small paper umbrellas - draw an outward-pointing arrow on the outside, then spin the umbrella. On a successful skill roll, the umbrella will stop spinning with the arrow pointed at the nearest demon within 10 yards. Should the user be himself a demon, he will be probably be the one that the arrow points at unless he first utters the ritual phrase "And not me, ye idjit." Should there be no demons within range, the umbrella will fold itself up.
This costs no Essence to use, and causes no disturbance.