How would you distinguish a necessary and unnecessary chemo after the fact?

Nobody would give chemo if the tumor is known to benign, after all.

An unnecessary chemo will be one where they couldn't tell, and the chemo ensures they wont tell.

The only way you'll know is from aggregate mortality statistics not budging.

The other aspect is that even if a tumour isn't, earlier detection is often not reducing mortality any more once you get to a decent baseline.

The evidence often does not support early screening in general. Early screening for specific subsets of the population have better results, but are also not unambiguously helpful.