I've had to have four dogs put to sleep in my life. Three to old age and one to illness.

I'm not an emotional guy, yet I bawled like a baby every time.

The dog will let you know. You will know they are letting you know.

Example of a final day:
My last dog (Copper - Australian Shepard) was 16, I let her out to go to the bathroom. It was snowing and we had about four inches of snow on the ground. She went across the street and couldn't get home. She just stood there. I whistled... nothing. I called to her... nothing... I went over and petted her... Nothing. I had to go carry her home. I knew it was time. When we were at the vet's office waiting to go in, she looked at me like she had so many times but this time, there was a finality in her eyes. A look that will be with me forever. A "thank you" look. A knowing look that her time was ending. When the vet gave her the injection that put her to sleep, it was... peaceful.

I have tears in my eyes as I am writing this. You will know when. I love dogs.