"Dearest reader, I humbly entreat you to eschew the latest celebrity tittle-tattle and instead devote your attention to diction and tone." Does that request sound funny to you? Maybe this sounds a bit more commonplace: "Listen up! Drop the gossip magazine and get with the diction/tone program!"
Why do these two comments sound different? Because the words come from two separate planets. The first comment resides in formal territory, and the second comment lives on the streets. Put the two side by side and you see contrasting diction — the vocabulary with which a writer expresses herself.
You were likely still in your cradle when you first understood tone — the way an author communicates a feeling or attitude toward the subject he is writing about. For example, in speaking terms, a sympathetic tone, as in "Oh, did you skin your knee? Poor baby!" can be distinguished from "Take out the garbage now!" without any formal lessons. Tone in writing is a little more difficult to determine, of course, because you can't hear the author's words as the author intended them to be read. Instead, you have to pick up clues from the text.
To determine tone in poetry (or in other writing), you have to consider diction and syntax, the grammatical structure of the sentence. You also have to consider which details are included and which are left out. For instance, if the author is listing reasons and answering likely objections in advance, the tone is argumentative or persuasive. If the poet goes on and on about the snowy, picture-perfect holidays of childhood, nostalgia is a good bet.
Here's a poem called "This Living Hand," written by John Keats:
This living hand, now warm and capable
Of earnest grasping, would, if it were cold
And in the icy silence of the tomb,
So haunt thy days and chill thy dreaming nights
That thou wouldst wish thine own heart dry of blood
So in my veins red life might stream again.
And thou be conscience-calmed — see here it is —
I hold it towards you.
To figure out the tone of "This Living Hand," you don't have to know that these were probably the last lines of poetry that Keats wrote before his death at age 26, nor that Keats knew he was dying. Take away those biographical facts, and you still see that this poem is both mournful and realistic in tone. The words are straightforward. For instance, the "icy silence" (line 3) contrasts with "red life" (line 6). The first five lines are almost brutal: This hand would "haunt" and "chill" (line 4) you. Keats isn't turning away in sentimentality; he's facing death, and he's making the reader face it, too, as he stretches out his still "living hand."
When you're determining tone, "hear" the poem in your head. Put yourself in the author's shoes and imagine what she feels. Examine the language closely, and bring your own experience to the poem.