There are a couple of reasons why Mrs. Salmon was so sure that the man she had seen was the accused man, Adams.

She does not claim that she has great eyesight, so that is not why she was so sure.  However, she points out that there was plenty of light from the moon and the street lamp.  These let her see the man clearly.

Next, she points out that the man's face is really quite distinctive.  That means that he is a man who would be very hard to forget and Mrs. Salmon can be confident she has the right man.