It is true, that you will know the correct time. That is hard to believe until you actually get there, though. But don't drive yourself crazy with self doubt. That will cause the suffering that you so much want to avoid. I have been in the situation before with more than one of my cats. Most of the time, my first clue to it being the right time, was a refusal to eat. Deciding to put an animal down is harder on the human than on the animal. If you are there with them and a nice familiar smelling towel is around them, they will think of you as their savior from pain (even on the vet's cold table).
What ever your decision, our thoughts and prayers are with you.