Once you've finished this draft, write a query and start sending it to agents.
If you get a lot of form rejections to your query, then you need to work on it. If your query gets you a lot of requests for the book, but form rejections for the book, that's not a good sign. After a certain number, enough time will have gone by so that you may have a fresh attitude about the book if you feel it warrants another rewrite.
If you're lucky enough to get real comments from agents, see if you notice any trends before you take it off the market. If everyone has a different reason for turning it down, keep submitting.
If you're lucky and talented enough to get an agent or an editor who wants you to revise, then you're in the upper echelons. If you can rewrite to their specs, you're in good shape.
If you do better than that and get it published, you're obviously home free--until the reviews come out, advising you that your perfect book will never be perfect in the eyes of everyone.