Killing Your Darlings

We're all supposed to do this. Go in with a blood-red pen and slash through all the words that we love--GOD how we love them--but which aren't doing what we need them to.

Maybe they're part of a hilarious scene that connects to nothing else in the larger work.

Or lyrical and challenging and lovely but the character they're describing/being spoken by should be cut from your MS.

Or they're pretty-good, and have one REALLY awesome metaphor buried somewhere in the creamy center, but the entire surrounding scene is just unnecessary, under-baked novel-dough. It's the incipient spare-tire of your novel. Verbal saddle-bags.


So yes, okay, we all know these exist.

But how do we know which darlings need murdered, and which ones are actually the very best you can do, and need to be enshrined, and worked in appropriately, and justly celebrated if you're EVER getting on any shortlists?

Which things are your "darlings" and which are just your absolute best writing?

I, for one, still haven't figured this out (which is why we have critique partners and writing groups, right?). 

But I really want to, because I have enough frigging neuroses already. Not knowing the difference between "I really love this because it's good and necessary" vs. "I really love this WHICH IS PROOF I SHOULD SLAY IT" is one I really wish I could put back on the shelf. 

Any tips, writer-land? How do you know when something's worth keeping or when, on the other hand, love just isn't enough?