Keep in mind that all poetry is meant to be heard, so it's important to read the words aloud as you are identifying literary tools and their locations. It's also a helpful way to recognize the uses, and it's fun when you're reading "Jabberwocky"!
Carroll uses words and the sounds made while saying them to create all sorts of effects. In the very first stanza of the poem, he starts out using nonsense words that sound like they should have actual meanings. The reader immediately starts imagining what these things look like as s/he reads, "'Twas brillig, and the slithy toves did gyre and gimble in the wabe."
Alliteration is present in phrases like, "Callooh! Callay!" Onomatopoeia is heard in the "snicker-snack" of the "vorpal blade". The rhyme scheme is maintained throughout the poem and adds to the musical quality of the words, even when the reader doesn't have familiarity with some of them.
And, as in uffish thought he stood, The Jabberwock, with eyes of flame, Came whiffling through the tulgey wood, And burbled as it came!
The threat, however, is clearly communicated in words that are easily understood and that sound as short and sharp as their message. "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!"