In Emily Dickinson's poem "I heard a Fly buzz—when I died—," we know that the speaker has prepared herself for death through both the situational descriptions provided in the poem and the poem's tone.
Based on the situation described in the poem, the speak is ready to pass on. There is silence and stillness in the room during her final moments. She is not crying out or protesting or struggling to stay alive. She is still, waiting. She has, apparently, accepted the fact that she will die. Those waiting with her have also resigned themselves to the inevitable. They have cried, but now their eyes are dry, and they are prepared for the "last Onset," just as the speaker herself is (line 7).
The speaker also relates that she has willed away her possessions, passing them on to others who can still use them. This, too, is a sign that she is prepared for death. She has detached herself from worldly things, letting them flow away from her and into others' hands.
The poem's tone also tells us that the speaker has prepared herself for death. It is calm and matter-of-fact. The speaker is not lamenting or grieving her exit from this life. She is not angry or upset. She merely states, simply and calmly, what she senses and what she has done. She hears a fly in the stillness. She is aware of the people around her. She has let go of her possessions. She is ready to greet death when it arrives.
Yet in the final stanza, there is a little hint that perhaps the speaker is still a bit apprehensive about what is happening to her. She notices that the fly's buzz is stumbling and uncertain, and this may reflect her own mindset. Even though she is prepared for death, her final passing remains a mystery yet to be encountered.