Two sections in Henry Wadsworth Longfellow's poem "Paul Revere's Ride" contain onomatopoeia, or words that sound like the sound they describe. Both sections describe the sounds the country animals make as Revere rides past the villages and farmsteads. In this passage: "He heard the crowing of the cock, / And the barking of the farmer's dog," the words "crowing" and "barking" are words that sound like a rooster and a dog. And in this passage, "He heard the bleating of the flock, / And the twitter of birds among the trees," the word "bleating" sounds like sheep, and the word "twitter" sounds like the calls of birds. Using onomatopoeia helps the author bring the scene alive for the reader; it is a type of sound imagery. 

Another word you might think is onomatopoeia that Longfellow uses several times in the poem is "tramp." This word refers to marching, and it makes us think of other words like "stomp," "limp," and "stamp." For some reason, words that end in /mp/ make us think of forcefully planting the feet down. This is a type of "sound symbolism" that is related to onomatopoeia but is a little different. 

How can you tell for sure if a word is onomatopoetic? You need to find its definition in a dictionary (online or print) that gives the origin of the word. If the word originates from another language, it is probably not onomatopoetic. But if you see the word "imitative" after the heading "origin," that means the word originated as an imitation of the sound it stands for, meaning it is onomatopoetic, as in the example definition of "bleat" linked below.