Omit needless words. Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts.
I know when a story is finished when there is not a single thing more I can think to do to it. And since I know at the start what the last line will be, I know when I’ve reached that point as logically as I can that it’s finished. As for the rewriting—it’s not foolproof, of course, but if you’re honest about having thought of every possibility and you still come back to what you have, what more can you do?