Because Trifles is a play instead of a prose story, it does not have a conventional "point of view" as such. The common terms of "third/first person-omniscient/limited" don't apply, because the audience only sees what is presented as action and dialogue. In that sense, the POV could be said to be third-person limited, since there is no narrator, and the audience can't know anything that the characters don't know. For example, when the canary is first discovered:
MRS. PETERS:It's the bird.
MRS. HALE: [Jumping up.] But, Mrs. Peters -- look at it! It's neck! Look at its neck! It's all -- other side to.
MRS. PETERS: Somebody -- wrung -- its -- neck. [Their eyes meet. A look of growing comprehension, of horror. Steps are heard outside.]
(Glaspell, Trifles, etext.virginia.edu)
In this scene, the audience only knows about the canary because it is in the dialogue. There is no description of it lying in the box for the women to see, nor is there any explanation of the meaning it has until they discuss it. In conventional prose, the narration -- either first or third-person -- would describe the box and canary, and then show reaction through description. With only the brief stage notes, the actors must make everything clear through their performance.
Another option is that the POV is of Mrs. Hale and Mrs. Peters, with whom the audience is meant to sympathize. Their perspective on events is very specific and informed by their knowledge of farm wives and women's issues; without their direct knowledge, the motive of the murder could easily be lost.