For starters, eating is a biggy for me. If a dog is not eating and they are typically a fabulous eater then it can be a huge sign. Of course not if they are just sick and stopped eating; I'm talking about the end of life not eating. Do they enjoy at least a fraction of their day? Do they really enjoy it? It truly is a horrible thing that we guardians are most often forced to do; make a decision on life. But as a dog owner it is our duty and the more care that we give our dogs the longer they live making it more imperative for us to decide.
I always say that I would be very happy if my very old dogs just slept away; I think most dog owners would like this. No decisions to be made, life just slips away when it is time. If our dogs were in the wild they would be long gone far before we ever even start considering that it might be time. Our dogs lives are literally in our hands; and that my friends can be a very sad thing. Taking the best of care that we can through their short lives is our pride and joy. Taking it away at the end is one of the most difficult things we will ever be faced with.
I think when you reach a point when the thought crosses your mind, it is our worst nightmare. We brush it away as a "what am I thinking?" moment.