How do you know?
How do you know if the cover they show you is OK? How do you know if the changes your editor asks for are the right ones? How do you know if the book you're slaving away on is actually worth finishing? In short, how do you know when to trust your gut, and when to trust the vast clamour of voices from Out There telling you what to do? Years ago someone told me to imagine making the opposite decision. You don't take the job, you don't buy the house, you don't have another baby. How does it feel? Can you live with the reverse? Is it more, or less, uncomfortable than the scary thing you're about to do? That's one test.
I heard Phillip Reeve speak a few months ago, and he mentioned giving up two thirds of the way through a book he'd been writing. This thought terrifies me, as I'm tempted to abandon every book I write at the two-thirds point. My second novel, Just In Case, was rejected by both my American and English editors in its first draft and went on to win the Carnegie Medal. Of course, they were right to reject it --it was barely a book, more of a sketch. My wise agent told me to keep working on it, and I do remember thinking that writing novels was far too time-consuming if you had to throw them away in the end. But the question remains, how do you know it's worth forging on to the finish?
I'm still not sure of the answer. I regret approving the paperback cover for The Bride's Farewell with the picture of a blonde girl. It's a lovely picture, but Pell should be dark. I'm not often swayed against my will, however, and the really wonderful feeling is when you phone up your editor or some other sensible reader, and say 'what do you think?' and they say 'scene X just doesn't really work for me,' and you feel a little jolt of shame. It's that little jolt that tells you what you knew all along -- that the damned scene never really did work, and you were just hoping to get away with it.
The best I can suggest is that you look for the little jolt. And always remember to thank the person who makes it happen.