Elizabeth...I ache for what you are going through now. It comes back all too vividly. Everyone has pretty much told you what I was going to say. I have had to do this 3 times in my life and my dogs were in their teens also when the decision was made. They gave me so much love and devotion and were so very pitiful in the end with so many ailments and not enough strength to fight much more. Yes, it is a horrible decision to have to make and one I do not want to have to make again for years and years and years! I do believe that on those final trips to the vet my dogs knew and I still saw nothing but love in their eyes along with the pain. I don't have a real answer other than to say that I agree that you will know and I believe your dog will know too.