In pre-civilization, how would they have known that someone had cancer?
Personally I am convinced that cancer is just one of many degenerative health issues. If you follow that line of reasoning, than the frequency of degenerative issues in skeletal remains are likely related to the frequency of cancer and any other degenerative health issue. As one example, if you simply see caries (cavities) in the remains of the teeth, that would also mean cancer was more likely to occur. (very healthy indigenous cultures have almost no cavities). Obviously this is very loose logic, and certain groups are more likely to have certain issues that are more or less likely to show up in skeletal remains, but I believe the general principle is sound.