How Can You Tell When It's GOOD Advice?

Yesterday I went to my writing group, where two of my fellow writers had submitted pages, and the rest of us told those (un?)lucky two what we thought.

Sometimes we agreed, which is always reassuring, both as a writer and a critiquer--it means there's probably something legitimate in the criticism.

And sometimes we didn't. At all. Or sometimes we offered wildly different potential solutions to perceived problems. Like mine was "maybe more dialogue" and someone else's was "if she passed out from pain, that would explain things"-level different.

This happens every time, regardless of who's on the block, because at the end of the day, writing (and reading) is about opinions.

Some of those opinions are so universally-shared as to approach the literary asymptote of factual "rightness."

(Let's pause a moment to pat me on the back for the phrase "literary asymptote," because I'm feeling pretty good about that one. Kay, thanks.)

Most of them are totally personal.

So how do you know the difference? Besides the obvious "what percentage of people who read this agree that X is good/awful/confusing/too heavy on cheese references" test?

And for argument's sake (and because I think the opinions of agents and editors take on special value for most authors, published or otherwise) let's add the caveat that you don't know the person offering an opinion well enough to know the objective value of his/her advice. You haven't read his or her writing, you haven't realized h/h particular hangups (which might wind up appearing too often or not often enough in critiques): it's coming at you cold. 

This is something I have always struggled with, and which, presumably, I'll continue to struggle with. I'd love to know what other writers use as a litmus test, especially when the problem--or proposed solution--is a biggie. Like, "gut somewhere in the thousands of words" biggie.

So? What's your gauge on when the advice you're receiving is good and when it really is "just one person's opinion?"