Trifles is not the first feminist play. For example, Ibsen's A Doll's House is a feminist work that predates Trifles by about 37 years. But Trifles is clearly a feminist work. One can justify this quite easily. The male characters take charge of the investigation because, as Court Attorney and Sheriff, Henderson and Henry Peters are simply doing their jobs. But they and Hale presume to think it is their role as men to take control of situations of such significance. They speak condescendingly to the women as they go about the business of their investigation. The men are stereotypical; they adhere to traditional notions of masculinity and male roles in society and in relation to women. Glaspell shows the stark contrast between these expected, traditional male roles and the roles expected of the women. The men are expected to be in command, assertive, and in charge of public affairs. The women are expected to be meek, subservient to men, and concerned with private, household affairs.
However, in this play, the women uncover the real physical evidence and in doing so, they discover Mrs. Wright's (some might say) 'justifiable' motives for killing her husband. While the men go about the business of the investigation, with the unearned self-importance that goes with their expected male roles, the women, in spite of being ignored by the men, become the investigators.
These stereotypical men assume that their stereotypical women are only concerned with the trifles in life and in the assessment of the case. Mrs. Hale and Mrs. Peters are not feminists in the modern sense, being overtly against such stereotypical roles. But they are feminists in the sense that they subvert what is expected of them within those roles. The women, not the men, discover the important evidence (the so-called "trifles") and in doing so, they discover how those rigid roles of dominant male and subservient female actually led to the crime. In the end, the women are in command of the situation. They decide to not reveal what they've uncovered in order to hide Mrs. Wright's motive. Half serious and half mocking ("in a false voice"), Mrs. Hale notes that the men might not even realize what their (the women's) evidence explains:
My, it's a good thing the men couldn't hear us. Wouldn't they just laugh! Getting all stirred up over a little thing like a—dead canary. As if that could have anything to do with—with—wouldn't they laugh!