Mrs. Bentley is considered to be a reliable narrator. Even though she tells her own story and there is a tendency to suspect that anyone telling their own story will have a limited and slanted perspective from which their ow good qualities get a bit better and the other characters' faults get worse, this can't be assumed to be the case because there are perceptive, astute, honest reporters of stories from the first person perspective. Some of the clues to Mrs. Bentley being a reliable narrator is that her story is told as diary entries; she tells her own negative thoughts and suspicions; she describes herself in stark terms; she is intelligent and was once a talented artist.
These clues are significant for the following reasons. Diaries are socially and historically accepted as reliable life accounts. This aura of reliability of diaries, which by their very nature are the writer's surrogate confidant, carries over to literature. So unless there are very strong clues that contradict the categorization of diaries as reliable primary records, a diary presents a clue for the narrator being likewise reliable. A clue opposing a reliable diary and narrator might be overt contradictions within the text, like changes in how another character or situation is described from time to time, or changes in how the writer's reactions to a type of event are reported from one instance to another (e.g., Jan3" I just adore parties where I don't know a soul. I meet such interesting people. May 14: I had to go to another party where I didn't know a soul. I was seeing red and breathing fire at those selfish fools as early as 10 PM.)
The others are significant because people don't usually reveal the unpleasant truth about themselves or their circumstances except in private. To others there is always a tendency to evade the unpleasant truth or to dress it up a little bit. An instance of this is that Mrs. Bentley describes herself honestly without pathos or melodrama and tells her worst fears and suspicions when she hears Judith laughing behind the shed. Further, her intelligence and previous life as a talented artist conveys that she has the breadth of mind and depth of perception into herself and other people that her assessments and opinions are not skewed by misinterpretation or pettiness or vanity. This all combine to make Mrs. Bentley a reliable narrator.