I'm going through something similar with my 15 year old Lab.
His joints don't work very well, and it takes him forever to climb up and down stairs. I will go upstairs for something, and the poor guy will follow me up. Then I'll be done doing what I'm doing and head back down just as he's getting to the top.
He's also going deaf. The doorbell he hears 50% of the time now. He used to go insane when he heard a doorbell on the TV.
Finally, the vet did an ultrasound, and he has a tennis ball sized tumor on his abdomen. You can see it bulging out.
I asked the vet when I would know it's time, and he said when he stops eating and drinking or is having lots of accidents in the house. He has pooped a couple of times in the house the last several months, but it's more from the fact that he doesn't want to deal with going out and walking down the deck stairs at night before we go to bed, so he does it in the middle of the night. Never when we're awake and home. Otherwise, he's old and slow, but he still eats everything in front of him and isn't laying around whining. We give him pain pills for his joints everyday in his food though.
My coworker recently put down his dog...also a Lab. He said his joints got so bad that he would have to hold up his hind legs to help him pee. That's when he knew it was time.
Either way, I'm basically in the same boat you are. Until my guy starts showing the loss of quality of life, I'm just helping him to enjoy his last days (extra treats, people food, extra time outside, etc.). When the time does come, it's going to suck, and it's going to be this year.