Pets: How do you know when it is time to put your dog down?

Aside from the obvious, like pain-how do you know. My dog is 14, a little crippled from arthritis, hard of hearing and demented ( wakes me up in middle of night ), pees in the house. I now have piddle pads everywhere. She takes mild tranquilizers so I can sleep. She is skinny, but still eats (although not much ) and loves her walks. I just don't want to wait until the last minute. She also has licking sores on her rear legs ( arthritic pain, maybe ). Ideas? Thanks. The vet was no help.