From Bulbapedia, the community-driven Pokémon encyclopedia.
The Aura Reader (Japanese: オーラサーチャー Aura Searcher) is a piece of equipment found only in Pokémon XD: Gale of Darkness. It is a scanner worn on the ear with a screen which extends in front of the eye. It is an attachment to the Snag Machine.
The Aura Reader does not play any role in snagging itself, but it plays a critical role beforehand. In a Pokémon battle, when the opponent sends out a Pokémon, the Aura Reader springs to life and identifies whether or not it has the dark aura of a Shadow Pokémon. Once the Shadow Pokémon is identified by the Aura Reader, the Snag Machine will allow its wearer to attempt to snag it (it will not function otherwise due to built-in failsafes).
The Aura Reader does not appear in Pokémon Colosseum, as Rui can see auras and identify Shadow Pokémon unassisted.