Sefer Chofetz Chaim chapter 7 halacha 9 footnotes 16, 17

The next more believable reason why one might think he can believe certain lashon hara is if it was told innocently, meaning “masiach lifi tumo”. The Chofetz Chaim comes out that though this is technically true, that it’s believable to be able to hate the guy whose being talked a out in one’s heart, but you certainly still shouldn’t tell others, and act upon it to cause him a physical or monetary loss because it’s still a degradation of your fellow Jew and even if it’s true you should judge favorably. However practically speaking, it’s very farfetched to have a real pure situation of “masiach lifi tumo” where you can trust what you hear to at least hate the guy talked about in your heart because there has to be no agenda whatsoever to the speaker. He had to have been just shouting and randomly a juicy piece of information which is lashon hara slipped out. A random example I had was if a non-Jew was telling over a story how he saw some guy with side locks, a long coat and a furry hat sitting down to eat a cheeseburger in McDonald’s and he was going on about how him and his friends were making fun of this guy for dressing so weird in the middle of the summer and what comes out of you listening to this is that you figure out a chasid you know ate non-kosher, that’s innocently talking. Now if you asked this non-Jew have you seen this guy around which you described as a chasid in those clothes and he said yes, I saw him in McDonald’s then certainly he’s not believed even if he had no intent of getting him into trouble, that not talking innocently anymore. Also, if you figure out that the speaker might have some agenda like to instill fear into you or to trick you and he’s acting like he’s talking casually without any agenda you still can’t believe him because maybe he’s just a good actor. Also, if let say the speaker was known to hate the guy that he is shouting about and he slips information which seems to be innocently said still it can’t be trusted, not even concerned about because we assume he always has an agenda since he’s an enemy. Innocent talk also certainly doesn’t work if it’s secondhand information because who knows if the first person who said it was talking innocently.