Anne Lamott always makes me chuckle. She equates finishing a book to tucking all the legs of an octopus and not letting them go. I hear this so often. "I think I'm finished with my book but how do I know?" I'm not sure there's an answer that suits everyone.
There are some questions to ask yourself. Are there any loose ends someone will find? I did this once to a friend who left a loose end that troubled me. The book was already published but when she had a chance to re-publish the book she got rid of that loose end. Have the characters reached a satisfactory ending? This is easy when writing a romance that finds the hero and heroine in each other's arms. In a mystery, the answer is two fold. Has the hero or heroine found justice and has the villain recieved that justice. There are a dozen other things to look at to make sure the story is over.
I usually tell people this. "When I feel like I'm going to vomit if I have to look at the story again I know it's done. It may not be perfect but perfect doesn't happen."
How about you? Do you have a special reason that tells you the story is completed and it's time to start a new adventure?