It's a fact of life that humans typically outlive their beloved pets. One of the more heart-wrenching decisions for any dog owner is knowing when to "let go," either because a dog is critically ill or old age is making life difficult if not unbearable.
A very informative article in The New York Times addresses the issue in objective fashion. The writer, Tara Parker-Pope, discusses her dilemma facing an end-of-life scenario with her own dog. She refers to a "decision tool" created by Dr. Alice Villalobos, a California veterinarian who is nationally known in the field of veterinary oncology. The tool includes seven indicators, such as "Hurt," "Hunger," and "Hydration," which a pet owner rates from 0 (very poor) to 10 (best) to determine the assess a pet's condition. Parker-Pope writes, "When pet owners approach end of life this way, they often are surprised at how much they can do to improve a pet’s quality of life... By revisiting the scale frequently, pet owners can better assess the quality of the pet’s hospice care and gauge an animal’s decline."
Dr. Villalobos tells Parker-Pope, "“Natural death, as much as many people wish it would happen, may not be kind and may not be easy and may not be peaceful. Most people would prefer to assure a peaceful passing. You’re just helping the pet separate from the pack just as he would have done in nature.”
You'll find the decision tool, as well as more helpful information, in the article here: https://www.nytimes.com/2019/03/19/well/family/pet-dog-cat-death-euthanasia.html?