An easy path for Apple to find data is simply to look for codepaths in web apps that are webkit-specific. They can take simple approaches like looking for usage of safari-specific CSS extensions are user agent sniffing, or invest more and actually profile running applications to observe unique codepaths taken on Safari vs. other browsers. I don't think anyone ever got to their keyboard and thought "I hope I get to re-implement my feature to work on Safari today!".