One thing to keep in mind is that unless you manage to eliminate all of the cancer by amputation or treatment, he will not get better.
It's been my experience that they let you know. Between the eyes, limping, etc, you'll know. I think I waited a day or two too long with Loca, and have always regretted it. My dogs were all between 11.5 - 13.5 years old when osteo hit, so it might have made the decision a bit easier.
It's a very, very tough decision. For me, I had to keep reminding myself to look after the dog's interest, not mine.