In battle
Frisk allows the Pokémon to see the opponent's held item upon entering battle.
If the foe is holding an item, a message telling which item the Pokémon is holding will be shown right after the Pokémon with Frisk Ability enters the battle. If the foe isn't holding an item, no message will appear.
In a Double Battle, an unidentified opponent is randomly chosen and their held item revealed.
Generation VI
In a Double Battle, Frisk now identifies the held items of all opposing Pokémon.
Outside of battle
Frisk has no effect outside of battle.