This is a good question. The basic idea is that you must not pass judgment on a person from hearsay. There are several reason for this.
First, the person who tells you about another person is biased. For example, he or she might not like that other person. If this is the case, then his or her opinion will be completely flawed. Let me give you an example from another area of life. Your friend might have hated a movie, but another friend might have loved that same movie. If you listened only to your first friend, you would not have had a balanced view. Moreover, you could end up loving the movie!
Second, people are deep. For this reason, you have to get to know people for yourself to form any opinions. This is a good rule of thumb.
Third, there are always two sides to all stories. For this reason, it will be important to take no one's word at face value. The wise and just person will listen to many others or better yet get to know the person directly.