Laws of Lashon Hara / 7: 1-2 (56)

1. It is forbidden to believe lashon hara even when told in front of a number of people.

2. It is forbidden to believe lashon hara even when told in the presence of the subject, even if the subject kept quiet which is contrary to his usual way of reacting, one cannot assume that his silence is an admission of his guilt It is possible that he reasoned that they were not likely to believe his denial, etc], and it is forbidden to believe it.