Literary devices is a broad category, but some examples are figurative language such as similes, metaphors and personification and rhetorical devices such as repetition and apostrophe. Douglass is very descriptive, and it does not take much to locate these examples.
Douglass uses figurative language often, to really help the horrors of slavery hit home. For example, he describes the overseer as having “stone-like coolness” (ch 4, p. 12).
In chapter 5, Douglass talks about the impact of going to Baltimore on his life by saying if he had not been removed from the plantation he would have “been confined in the galling chains of slavery” (ch 5, p. 15). He uses this metaphor a lot.
In chapter 10, Douglass addresses the ships in the harbor, comparing them to his own life. This is known as apostrophe, which is when a character talks to someone not there.
“You are loosed from your moorings, and are free; I am fast in my chains, and am a slave! You move merrily before the gentle gale, and I sadly before the bloody whip!” (ch 10, p. 29)
They can leave the harbor, and be free on the ocean, and he never can be free of slavery.
In this same chapter Douglass uses another device he uses frequently, repetition.
O that I were free! O, that I were on one of your gallant decks, and under your protecting wing! ... O, why was I born a man, of whom to make a brute! (ch 10, p. 29)
By repeating the exclamation, he really brings home his feelings and what he was thinking at the time.
Also from chapter 10 is this simile. A simile compares two unlike things.
When I could stand no longer, I fell, and felt as if held down by an immense weight. (p. 29)
In this case, Frederick compares his body to a weight. He has been weighed down by slavery. He uses this language again in the same chapter.
The white men were on horseback, and the colored ones were walking behind, as if tied. (ch 10, p. 37)
In chapter 11, Douglass uses both repetition and simile to compare how he feels about people who think they understand slavery.
—in the midst of houses, yet having no home,—among fellow-men, yet feeling as if in the midst of wild beasts ...(ch 11, p. 43)
By describing what is happening to him with literary devices, we are able to really picture what he went through. As Douglass says, we can never really understand. Yet his frank, honest, and precise descriptions are so vivid that he lets us into his world a little at a time, and we are grateful for it.