Ok here's the second post in a series inspired by this post on DOA.
Now judgement is a hard subject to talk about as it is, by it's very nature, a subjective topic. I guess to be able to judge effectively about your mix you need to know what sound you are going for - if you don't then this will make certain mix decisions very tricky.
Once you know what results you are after then you need to ask yourself "does this sound good?", "is this right yet?" and "if it's not right yet, what's wrong with it and how can I get it to the point that I want it to be at?"
One of my music tech tutors in college once said to me: a tune is never done, but after a certain point you need to draw a line under it.
Sometimes you need to be able to recognize when a certain synth patch or sample just isn't good enough and no amount of processing will ever get it to where you want it to be; hell, sometimes you need to recognize when a tune is always just going to be a 8/16 bar loop.
Judgement cannot be taught and you can't really write advice about judgement. It comes with time and experience (the better I get at production the more I am convinced that your favorite producer x with the fantastic mixdowns and sound design has no production secrets; just a really good knowledge of their tools and a highly developed sense of judgement).
I don't think there's any more to say on this.