Mrs. Hale is a reliable narrator in Susan Glaspell's Trifles, though she certainly ends up acting as an advocate for Minnie. She is both observant and intuitive, making her an effective and reliable storyteller for us, though her husband and the other two men do not benefit from her observations.
Mrs. Hale and Mrs. Peters are only here at Minnie's house to do what is deemed byt their husbands to be "woman's work," gathering some things for Minnie while she remains in jail for murdering her husband. The men are here to do the more important work, gathering evidence about the murder. As the men go about their jobs, the women do nothing but what women would do in these circumstances: they tidy up the house and notice things the men see but overlook as evidence.
The women give meaning to the broken fruit jars on the counter, the crooked stitches in the quilt, and the dead bird. They piece together a very unhappy life for the young girl. Minnie loved to sing but her song had been stifled, just like her canary. Her husband would not put in a telephone for her, preferring to keep her isolated and trapped. Her husband was a man capable of violence (as demonstrated by the wrung neck of the bird), but mostly he was just an insensitive, selfish, and rather brutish man who was unconcerned about Minnie's feelings, wishes, or desires. None of these bits of evidence in the house is particularly disputable, and the women's observations about Minnie complete the picture.
The women conclude that Minnie did kill her husband, which may be the same conclusion the men will eventually draw; the difference is that the women will understand what prompted her to do it. The fact that they choose to hide some of the evidence to protect Minnie (knowing that male authorities and a male jury would never empathize or understand Minnie's motives) is the only thing that makes us suspect Mrs. Hale's reliability. In every other area we trust her.
When she says that the men discount women's opinions, we know she is right because we hear them say such patronizing and dismissive things as this:
"Well, women are used to worrying over trifles.... And yet, for all their worries, what would we do without the ladies?"
When she suggests that the men do not understand what they see, we hear it in their conversation. For example, when the county attorney disparages Minnie's housekeeping, he assumes it is because she is a slovenly housekeeper:
"Dirty towels! [Kicks his foot against the pans under the sink.] Not much of a housekeeper, would you say, ladies?"
Mrs. Hale looks at the same evidence and sees it this way:
"Those towels get dirty awful quick. Men's hands aren't always as clean as they might be."
Clearly Mrs. Hale is reliable in that she sees everything--and more--that the men see. The difference is her interpretation of the facts. Though she certainly has a bias toward Minnie and her unfortunate circumstances, Mrs. Hale faithfully reports to us what she sees, though of course she does not share everything she learns and knows with the authorities.