Shemiras Halashon Halachos

Hilchos Rechilus, chapter 6, section 8 9

Even if the person said the rechilus incidentally while speaking of other matters (meisi'ach lefi tumo) one may still not believe him.

Hilchos Rechilus, chapter 6, section 10

If there are clear signs (devarim hanikarim) that the rechilus one heard, that so and so spoke or did something against one, is true one may believe it. But this is only if there is no way to judge these clear signs favorably, for if not, even if rechilus itself is perfectly true one must judge the story favorably if possible. The signs must be absolutely clear and not only slightly clear. One must have seen the signs oneself and not merely heard them from others. Even then, there must be some future benefit in listening to the rechilus because otherwise one may not listen to it in the first place. Even if there are clear signs, one may only believe the story as far as oneself is concerned. One may not relate it to others. Also, one may not rely on the clear signs to cause the person a monetary loss or to strike him.