The React developer tools usually can tell if a site is using React and shows you the component hierarchy. Screenshot of https://developer.apple.com/documentation/ taken today: https://i.imgur.com/yF1Q3uY.png.

The react devtools extensions for Chrome usually shows me when a page is using react.

Right click -> view source?

I did that, but I don't know what to look for. I searched for React but it didn't find anything. The article claims Apple uses React. I wanted to verify that claim.