You know it's done, a painter once told Gertrude Stein, when there's a frame around it.
Painters are lucky that way.
Poets often won't let a poem be, even after it has appeared in a magazine or book. Take W.H. Auden for instance. He fiddled with his poems each time they were reprinted. You know a poem is finished, we have to say in some cases, when the poet is safely underground. Still, of course, there are always the notebooks.