Exposition is revealed when Hale describes how he looked in on the Wrights to ask about a phone. The first complication, which is the signal for the rising action, comes when the audience hears him ask why he can’t see John, and Mrs. Wright answers “because he’s dead” (Although this is told in retrospect, which makes it exposition, the telling to the sheriff is the complication.)
The action rises throughout the play steadily (this is virtually always the case in a short, one-act play) but we could say that the next “beat” begins when the men go upstairs. The turning point is the discovery of the bird wrapped in a cloth. And the climax of the dramatic structure and tension occurs when the bird is intentionally concealed from the sheriff.
These technical terms to describe the parts of dramatic action are more suited to the “well-made” play as built and dissected by Eugene Scribe in the 19th century; “Trifles” is not only shorter, but is also a product of the emerging modern play movement, such as the naturalistic plays of Eugene O’Neill, and, in Europe, Ibsen, Chekhov, etc.