There are times when halacha accepts as fact “innocent” remarks, i.e. statements made in passing without the speaker being aware of the consequences of his words. As a rule, people do not lie unnecessarily, so that when a person does not realize that he is saying something of significance, it can be assumed that he is telling the truth.
Halacha grants such statements the status of testimony in two instances:
1) Where it is certain that ultimately the true story will be revealed to all. People do not fabricate tales when it is clear that their lies will be revealed as such.
2) To establish that an individual is no longer alive.
“Innocent” negative statements are not granted any such legitimacy. They are loshon hora and one is not permitted to believe them.