How do I know when it is time to put my dog down?

I have a 13 year old lab with hip issues. Her hind legs will give away if she is on tile or any sleek surface. She seems to have moor "accidents" than ever and her hearing is not the greatest. But, she eats, goes for short walks (starts panting after 1/2 hour) and sleeps well. My husband thinks she is in a lot of pain because her tail is in between her legs a lot and she has a hard time getting up sometimes, but I am not sure.