Impossible to know because they can't speak for themselves and they really don't know about life and death, only us humans are cursed having to know we are going to die.
My 16 year old golden I kept alive right to the bitter end, for 4 months I nursed her round the clock. The vet knew I didn't want to let her go without giving her a chance to recover, he told me she wasn't suffering too much so I kept her going - another vet in the same office said she should be put down, that there was very little chance she would improve. They never told me or knew what was wrong with her, it wasn't cancer or heart, it was something neurological and it happened so suddenly. I saw it happen, told them exactly what I had seen and they couldn't tell me what it was. Weird.
Unless the dog is really suffering, there are no good moments in his day, I would keep him alive. People who have cancer suffer but we don't put them down, and the vast majority of people wouldn't take euthanasia as an out - UNTIL they are in total misery without any hope for a better day. I definitely believe in euthansia for animals and people.
It sounds to me like your dog still has wants to be here, despite being uncomfortable he still is living life. When it's obvious to you the pain and discomfort has taken away any pleasure from his life then it's time to say goodbye
In my dog's last hours, I have no idea what happened, I hadn't seen her in a few hours, was working on a website I remember. I came downstairs and couldn't find her in her regular spot, she was barely mobile at all so i was alarmed, i ran all around looking where she might have gone, I found her under a desk that had a bunch of stuff under it, could only see part of her - I saw this pile of red fur, I approached it scared and upset, whatever happened, her body was folded up in a very unnatural position and I bent down to see if she was alive - she was but obviously something was very wrong and she was definitely dying. I frantically called the vet and my sister, he told me not to bother bringing her in, he would come over and put her down. So I had about half an hour to sit with her and comfort her - she was aware. Fuck, it's 6 years ago and my eyes are tearing up. I went to the kitchen and got one of her favorite foods, Twinkies, incredibly enough I put the Twinkie to her mouth and she snapped at it and wolfed it down, vanilla cream all over her face.