The prohibition of accepting loshon hora does not apply when the subject is known to regularly engage in the negative behavior under discussion. Thus, if on a number of occasions one has personally witnessed an individual commit a particular sin, he would be permitted to believe that the person committed that sin again. The Chofetz Chaim seems to suggest that one would even be permitted to believe the person has committed a different sin. When a person is known to totally disregard the Torah and its mitzvos one may certainly accept a report that he committed any sin.
If one is known to regularly transgress a particular sin out of temptation, one would be allowed to believe that he transgressed it again, but would be required to reject a claim that this time it was done maliciously.