Clea Simon (she has a great blog, cats, crime and rock and roll) asked a great question about when you know a book is done. For me, it usually has to do with feeling. I can't think of anyplace else to go with it. I've read it so many times, and shown it to enough people that I can't find any openings to even add in an adjective. This is when I use my outlines and synopsis and character mapping--that lets me see if I have any holes in the story. Of course, a novel is never really done. You can always think of more things that you wished you had changed, can't you?
For me, though, a lot has to do with my next project. It starts taking on more life and crowds out the old project, bit by bit.