Today's Chofetz Chaim Halachos

Even just listening to lashon hara is forbidden by the Torah, even if at the time the listener does not plan to believe it, because he is turning his ear to it.

But listening and believing are different in some ways. Listening is not forbidden if the story matters to him for the future. For example: right at the start he understands that the speaker is about to say that a certain person is not honest, and he was thinking of doing business with that person, or of making a shidduch (marriage match) with him. In such a case he may listen, so that he can be careful about that person. His goal is not to hear bad things about someone, but to protect himself from harm.

He may also listen if some good can come from it. For example, he plans to check if the story is true, and if it is, to correct the person. That way the person might stop sinning, return what he stole, or make peace with someone he insulted.

But all of this only allows listening. To accept the story and decide that it is true is forbidden.

The reader should not wonder: "If it is forbidden to hear bad things about others, how can a person do the right thing? How can he check out a business partner?"

The answer: someone who wants to do the right thing should act like this. If a person comes to tell him something bad about someone, he should ask him whether this will help him with something in the future. If the speaker says it will help, he may listen, but not believe. He may only treat it as a warning and be careful. But if he understands from the answer that it is just insults and curses, and that the speaker is making up accusations out of hatred, it is forbidden even to listen.