Chris,
Sorry you are having to go through this. I know how tough it is to have to make this decision. I went through it with both of my dalmatians.
I see datawiz already suggested doggie diapers. I had to use those on Apollo his last few months because he had difficulty controlling his bowels sometimes. They do work as a short term solution as long as the dog doesn't chew them off.
As far as knowing when the time has come, I think it should be when the dog no longer has a good quality of life. You will most likely be able to tell by her eyes when the time has come.