Hi Ned. I think the short answer to your question of how you can distinguish back pain due to bone mets from back pain due to injury/degenerative processes is that sometimes you can't. Here's how I came to that conclusion:
Background (no pun intended): Chronic lower back pain (like since high school!), finally had back surgery in 2000 to fuse the L4-L5 and L5-S1 joints. Surgery was botched and of no help but I learned to stop complaining.
Fast forward: Dx 12/07 with NSCLC with mets to pelvis (beginning precisely where they'd harvested bone in 2000 for the back surgery, hmm, rear left iliac crest) but it didn't hurt at all. About 2 months ago, I felt pain in the right groin and hip and they found a met on the right side of the pelvis hitting the top of the femur. Then I developed good old sciatica -- shooting pain down the right leg, sensations of burning and stabbing in right leg and foot. This I was sure was not from a met since it felt like the sciatica I've been prone to lo these many years. Well, I was wrong. A third met had formed on the L3 vertebra, on the 'inside', facing the spinal cord (actually cauda equina at that level) and poking into it. And it was the L4 nerve root being poked, which doesn't exit till the L4-L5 space. So it was the same nerve root that I'd always had problems with, only this time caused by a bone met instead of a degenerated disk, and higher up in the spinal column than the disk was.
I agreed to 14 radiation tx to nuke the right side of the pelvis and the L3 vertebra. The L3 vertebra problem disappeared the second week of tx and has stayed away; the right hip/groin still hurts like hell but I'm told the radiation could still do its thing.
Anyway, the lesson for me is that all pain is something impacting somehow on some nerve and the affected nerve doesn't care whether it's a tumor or a piece of spinal bone collapsing on it or a bullet or whatever -- it just hurts. Btw, my original met (left rear) is now quite huge but is still totally painless -- not near any nerves or moving parts.
So maybe mention the problem to your onc. Just stay away from neurosurgeons no matter what it is. In the meantime, you might want to try large doses of Ibuprofen (800 mg at a time) -- it works pretty well for me whereas Oxycontin etc do nothing.
Good luck! And aloha from Philadelphia.