In Part I, Bierce uses Farquhar's watch as a symbol of the doomed man's mortality. Clocks often symbolize the inevitability of death, and this story's watch is no exception. The narrator even says that the ticking of the watch "was regular, but as slow as the tolling of a death knell." Then, the time between the watch's ticks grows longer and longer, and Farquhar's apprehension grows as the delays become "maddening." As time feels as though it is slowing down, Farquhar seems to sense that the inevitable can only be delayed, and the waiting for it -- for death -- makes it so much more tense. In this way, Bierce uses the ticking watch to symbolize the inevitability of Farquhar's death as well as to build tension.
Further, the narrator says that the ticking of the watch that counts Farquhar's remaining seconds "hurt his ear like the thrust of a knife." Thus, Bierce uses a simile to compare the effect of the sound made by a symbol of death to an action that also causes death. He's doubling the death with such a comparison. Both the symbol of the watch and this simile serve as clues that Farquhar cannot possibly survive this episode.
In section III of the story, the action takes place in Farquhar's mind in the instant he falls from the bridge at the end of the rope. The reader, however, is led to believe that Farquhar's dream of escape is reality. Yet, as with all good twist endings, the author leaves clues to suggest the truth. These clues involve Farquhar's mental function, his physical sensations, and the scenery.
First, at the end of section I, Bierce writes, "these thoughts ... were flashed into the doomed man's brain." Section II interrupts, so the reader forgets that statement. But section III starts by saying the man lost consciousness. When Bierce says he was awakened, he adds the caveat "it seemed to him." Later he writes, "the power of thought was restored; he knew that the rope had broken." Indeed, it is the power of thought that creates his whole escape, and what we read next is what "he knew," not what was really happening. The succeeding descriptions of his "preternaturally keen and alert" senses are definitely more dreamlike than realistic. He sees the veins in each leaf on the trees, the faraway soldiers are "gigantic," and he sees the gray eye of the marksman. Indeed, we are told "his brain was as energetic as his arms and legs," which we know afterward was certainly the case.
Next, the sensory descriptions point to what is really happening but are reinterpreted through his dream state. The heat of the bullets against his neck, the water that "strangled him," the fact that he feels himself "spinning like a top," and his feelings of neck pain, eyes being congested, and tongue swollen are all what is really happening, but his brain works them into the action of his imaginary escape.
Finally, descriptions of setting starting in the seventh paragraph from the end are typical of "near death" experience reports and literary descriptions of death. The "roseate light" shining through the trees, the sound of "aeolian harps," walking down a seemingly endless path that has no signs of civilization, seeing the stars in unknown constellations, and not being able to feel the road beneath his feet are all consistent with a description of a journey to the afterlife. In the penultimate paragraph, the tense changes from past to present and a time warp is suggested as he feels he has fallen asleep while walking. He sees his home and his wife in the "morning sunshine," the final vision his brain chooses to focus on just before the end.
Although Bierce convinces most readers that Farquhar's imaginary escape is real until the final sentence, he leaves plenty of clues that satisfy the reader when looking back, making the twist ending of this story particularly effective.