Well, I know when I've written a script that was so poorly conceived it couldn't be saved with any number of rewrites -- which is most of them so far.

One piece of advice I've heard that I think bears repeating is that if you're at the stage where you keep taking things out of the script and then putting them back in, you're probably done.

I haven't hit that point yet with any of my scripts. When I do rewrites, I'm still revamping huge story elements and characters. But such is learning.