Hilchos Lashon Hara chapter 7, section 5
All this applies if the victim is a regular Jew. However, one may believe lashon hara said of a person already presumed wicked due it being publicized many times that he deliberately transgressed well known prohibitions such as adultery.
Hilchos Lashon Hara chapter 7, section 6
If someone speaks of his personal matters and includes something derogatory to himself and someone else, one may only believe the story regarding himself and not the other person.
