Using this Gift, a Theurge can roughly identify a spirit by its airt -- the trail left in the wake of a spirit's passing. This works essentially like tracking in the corporeal world, and is no more informative -- a hunter can tell deer tracks from bison tracks, but can't learn anything meaningful about an unknown creature. Also, powerful and subtle spirits are often able to disguise their airts. Any ancestor-spirit renowned as a great hunter can teach this Gift.
Treat this exactly as a Garou identifying and tracking animals (by scent or by looking for tracks, at the Theurge's discretion), but apply it to spirits instead. Note that Garou may not be able to go everywhere spirits do to follow the trail -- remember, spirits can fly.