The Boo Radar will blink yellow as there's a Boo in this room. If Luigi hits an object, it will be a Boo Ball, Bomb or Boo. If Luigi finds a Boo Ball, Luigi can blast it away with his Poltergust 3000. If Luigi finds a Bomb, it will explode and Luigi must avoid it. If Luigi finds a Boo, it has some HP and touching it hurts him and the Boo will go into a location that's cleared. If a location isn't cleared with a Boo, there are two Boos. Getting all of the Boos makes Luigi get a Diamond.
If Luigi does nothing in a location with a Boo, Luigi hears the sound of the Boo, as the Boo is in the location.