(if anyone's still there)
my answer would be that you dont. that is, hacks being naturally ignorant, they generally dont have a very good perspective on their work. this is why they continue to go on and on.
the difference between a hack and someone who is worried that they might be a hack is that usually a hack isnt worried about being a hack. they are that disconnected wih reality. also, the more well-read you are and the more experienced you are, the less likely you are to be a hack, because you are generally more careful with your screaming. it's entirely possible, for instance, that i am a hack. i mean, i've posted quite a few things here and on my website. it's possible that i'm completely misguided about my own talent, as humble as i look at it. however, there is a difference between me today and me 10 years ago, and one difference is that i'm a bit more well read than i was 10 years ago, and 10 years ago, because i was at least perceptive enough to recognize my own limitations, the last thing i would have considered doing would have been shouting about my small canon of work.
i guess there is a degree of self-reflection required in order to avoid being a hack. coincidentally, this self-reflection is precisely one of the things that is required for someone to not be a hack. so it all works out.
the bottom line is, if someone stinks but they refuse to see it, there's not much you can do other than simply acknowledge to yourself their irrelevence. you probably wont convince them of anything. but if a writer is worried themselves about being a hack, that worry in and of itself is an indication that they are selfconscious enough to avoid being a hack, which, again, is someone who is loud and ignorant.