1. When you want to clean out your refrigerator more than wanting to do another read-through of your WIP.
2. When you decide to suck it up and read through it just one more time anyway and realize, "Hey. It doesn't half-suck."
3. When your CP keeps telling you it's good. Don't worry. And you trust your CP.
4. When you start going through Google Images to envision what your book cover might look like.
5. When you are actually looking forward to sending it to your Agent, knowing it is the best you can make it and are actually hoping for some tough criticism so you can make it even better.
6. When you lay in bed at night thinking through the story and characters and seeing the book play out like a complete movie in your mind. And liking it.
7. When the "Other Idea" starts to push into your consciousness with more and more force and taking up more space because you are no longer working through plot problems with your current WIP.
8. When the only substantial revisions you seem to be doing involve changing an "And" to a "But". And then changing it back again.
9. When you can think of nothing more to make it better. And tinkering with it any more might make it worse.
10. When you realize that a book is not going to become a book unless you let others read it. Otherwise it's only a diary.
That's when you know.