When it says a person is believed like two witnesses what that means is that on a personal level you can trust like a confidant that he would never lie to you. Even if he is known in the world as an honest person, he is the gadol hador, still if you don’t know him personally you aren’t allowed to trust what he says when speaks lashon hara, as if he is two witnesses. This is evident from the Mahari”k quoting a gemara in Kesubos where Rava didn’t trust Rav Pappa about discrediting a document though he would have trusted Rav Chisda’s daughter. Though Rav Pappa was a great Amora, and he had many dealings with him as we see throughout the gemara, still in all Rava didn’t feel he knew Rav Pappa enough to treat him as two witnesses. The person has to be trusted by you personally as if he is 2 witnesses testifying in court not just two people. This means that you have to personally know he wouldn’t even add or detract one word from what he hear from someone else. There also has to no angle of innocence for the person talked about and it has to be said for a beneficial reason. With all these factors the Chofetz Chaim says that it’s impossible now a days for a person to believe anyone like two witnesses. That is what the poskim have poskined, name the Ri”f and R”osh, Rebbeim that lived over 800 years ago. People unfortunately make excuses to themselves that they can trust their parents or spouse, that they would never lie to them, but the Chofetz Chaim said that even the most modest of women, it’s one in thousand chance that they don’t leave out at one word or add some detail, and there for even the closest family member cannot be trusted like 2 witnesses. The only thing you can do now a days is be concerned of what you hear but never accept any lashon hara or rechilus as truth.