The decision should always be guided by concern for your pet's quality of life. Vets use a tool called a 'Quality of Life Scale' or the 'Hurt Scale,' which can help in assessing if an animal is suffering. The scale involves multiple factors like pain, feeding habits, and more. Always consult a vet in this process as well, as they can guide you in making the right decision - based on the health, comfort, and well-being of your companion.