Could someone please ELI5 how one proves that he was not hired for an engineering position "for which she was qualified" because of one's age? From my experien

Could someone please ELI5 how one proves that he was not hired for an engineering position "for which she was qualified" because of one's age? From my experience there are usually multiple candidates that are perfectly qualified for any opened position but only one (the best?) is hired. What kind of evidence could we expect to see in a case like this?