A conundrum! Whenever I hear this question, I think of the poem W.S. Merwin wrote (“Berryman”) about asking John Berryman a similar question: “How can you ever be sure / that what you write is really / any good at all and he said you can’t // you can never be sure / you die without knowing / … if you have to be sure don’t write”.
Sometimes I feel a poem is finished when I write a satisfying final line; but that can be misleading, as I may later realize that line is a kind of summing up that the poem doesn’t need. I often feel I say too much in a poem, and could pull back, so that’s often a revision strategy for me. After workshopping and revision and massaging and re-ordering and fretting about word choice and line breaks and punctuation, the uncertainty doesn’t disappear, but it does diminish. If I’m lucky, there is a moment when I find myself physically nodding at the poem—a kind of greeting, a moment of recognition.