You can’t know by individual stories. But there have been 11 Randomized trials performed that proved that screening saves lives beginning at age 40, even in the days of primitive mammography. https://twitter.com/susan_bewley/status/1252709812489551873

Modern observational studies show the benefit with modern equipment. This study of almost 3,000,000 women in Canada, showed that women who have screening mammograms are 40 to 44% less likely to die of breast cancer, then those who don’t. https://www.ncbi.nlm.nih.gov/pubmed/25274578 

And this study from Sweden shows that women who have mammograms are 60% less likely to die in the first 10 years after diagnosis , and 47% less likely to die after 20 years, than women who don’t. https://pubmed.ncbi.nlm.nih.gov/30411328/