I mean...I guess, depending on what illness they might have. Typically they will become lethargic and stop eating, stop being social. If they don't have quality of life (they don't find joy in anything anymore) then probably it's time to put them to sleep. My girl Chloe, had a very aggressive cancer. You could see that she had become anemic because the color of her eyelids, and her gums had gone from pink to almost white. She wasn't happy with anything that used to make her happy, she just started to refuse food...even "people food" that she loved, so it was time. Hardest decision I've ever had to make in my life! My husband was in denial about it too, so it was also my job to convince him that she was ready. I'm sorry to hear it, if your pup is not doing well.