The question in the title of this post is one that I am often asked in interviews, by other poets, and by readers of poetry. I can only speak from my own experience.
When one goes to an eye doctor to have one’s eyes checked, there is a machine in which lenses of various strengths are flipped back and forth and one reads a line of letters of the alphabet. The doctor says “Which is better? this one (A)? or this one (B)?” One way to know that the correct lens strength has been reached is when a newly flipped lens is too strong, the letters so sharp they blur, and the patient says “go back to the one just before this one.”
I find that something similar is true when I have gone through several drafts of a poem. Near the end of these revisions, I am usually down to single-word or single punctuation mark changes here and there. When I get to the point that I repeatedly “go back” to the word or punctuation mark I had just before — because I have made a change that is weaker than what preceded it — then I know that the poem has probably reached its final form, or at least has come as close to such form as I have the power to make it.
Another way to know a poem is “done” is when one has read and reread the poem many times both silently and aloud, and the syntax, grammar, rhythm, rhyme, symbolic imagery, etc. all seem to come together as one in a kind of seamless weave. When this happens, the poet feels a deep sense of relief and peace because the poem is a “live birth” and will live on its own now apart from its maker. The poet feels something like a religious “grace” experience in relation to whatever he or she considers to be the meaning of the name for poetic inspiration — the “Muse.”
Going back to a poem after weeks, months, or years and making changes is tricky and can be dangerous. One can get so far away from the poem that one is tempted to revise it into what is essentially a new poem reflective of a later phase of or stage in one’s life as a person and a poet. Some poets are known for making major revisions in earlier work — Yeats and Auden, for example. I myself never change a poem that I have been away from for, let us say, a year or more unless some glaring local error that really must be corrected presents itself to me for the first time. As a rule, it seems better to write a new poem on the same subject than to keep “revising” the record of one’s past. Speaking as one who was young in the 1960s, I don’t intend to change, poetically speaking, a paisley shirt and bellbottoms into a coat-and-tie and a pair of dress slacks in the “wardrobe” of my earlier poetry. As the Beatles said, “Let It Be.”