Timber Tricky one,be great if we could see into the future wouldn't it?Who knows maybe when the time comes,your dog might just sleep on,like some do and you won't have to make that decision.
Remember a few dogs we grew up with,belonging to neighbours and one of our own,all in their twilight years.Each in turn took themselves off when it was their time...went off to die,some say.I think they know when they've had enough and find a way to let us know.(( hugs))

There is an old saying I was taught by the farmers dad before he passed away,
I would for months see him standing in a field with his 2 old lurches, brothers they were and the 3 of them would stand and stare even though one of the dogs was blind,
I asked him what they were doing and he simply told me that they were remembering old times from when the 3 of them used to hunt rabbits, he'd tell story's and they would stand and listen and wag and lick,
Both of those dogs died in their sleep on the same day hours apart, never did they seem old and he said it's because they spent so much time remembering the past that they didn't have time to get old, he didn't leave the house after their passing and spent every day of his last 3 months with a tear rolling Down his cheek,
The saying he lived by was
" A dog, an old dog can't look at pictures he has only his memory's, and an old dog standing and staring is remember life as a young fit dog, you can't see what he remembers but he would be greatful for you to stand with him and remember yourself, the fun that you shared"

I just wanted to clarify that disabled is quite different from pain. We took in a cat several years ago. She was grown but kitten sized with severe neurological issues.

We never considered euthanizing her. She wasn't in pain, could feed herself and crawl into the litter box. Her walk was kind of a step step shuffle shuffle fall over and get up again. We had to bathe her daily and trim her claws weekly. But she was physically healthy actually and passed from natural causes a bit more than a year later.

But as another poster said with any animal, especially one you've known a long time...the eyes will tell you when they're hurting too much to go on.

I completely agree that disabled is different from pain. I don't know if the pacing is because of pain. It takes a long time to settle sometimes, and i'm not sure if that is pain related either. I saw a dog at the brink of death from kidney failure and it was so sad.