Re: When do you know?
Yeah, I don't think it gets easier to "know", as such, I think you just get to a point where you become a little more confident in your abilities. Couple that with the knowledge if you want to attempt to get it published then other people will have their own opinions on it, and I think you just get on with it then.
It helps me to know I have other projects I will continue with. It reminds me the current story isn't the be-all, which frankly when you are right in the middle of writing it does feel like it.
Post some in the Critiques section here, then see if you can find some readers (trust me, these people are invaluable. Hold onto them where you can) - they will at least flag up anything obvious for you.
Also, I agree with what Teresa said about it all being part of your individual learning process. Be proud you have written as much as you have (many people don't even get that far).