I think animals usually are pretty good at telling us when they're ready to go. We had a dog who when old was really not in the best of health. We had him in and out of the vet's office getting meds because he kept becoming anemic (not enough red blood cells to properly oxygenate his body) and we kept giving him stuff that was I guess supposed to help his body produce red cells or something. One of the times he really wasn't responding and not eating (unless I held a handful of food to his muzzle and wouldn't let him leave until he ate it, of course I think he was eating more for me than for him). He had to be carried outside usually, and if he did go out on his own, had to certainly be carried back in because he just went out and laid right down, seemingly very weak.
My mom was out of town, so, although we had some phone contact (no cell phones for us yet, so we only talked to her if she called) it was pretty much my dad and I left to making the decision. We decided one morning that if he hadn't turned the corner by the time my dad got home from work, we would take him in to put him down. I was on summer vacation, so I was home with him. I went to try to feed him, and to my surprise, he not only ate the food in my hand but ate seemingly somewhat enthusiastically out of the bowl! I then went to see if he wanted to go outside. Not only did he go out without help, but he walked around, did his business, and came in without needing assistance. I think it may have been his way of telling us he wasn't quite ready to go just yet, even though from where we were standing his quality of life was quite poor except for that time.
My dad and I decided to wait and watch another 24 hours in light of this development. Unfortunately, though, 24 hours later saw him going downhill again and we just kind of got the feeling he was ready to go. But I'm glad we waited until then.