If you have the React debug browser extension it lights up when it detects react. There is a way for the project to not as its supposed to be only for development.

Anyways, I'm a huge fan and supporter of React.

However, sadly, you can tell sites that use React due to a few bugs.

The most noticeable is/was funnyness with input box focus and state. Facebook's embedded Messenger on web always bugs out on me by not accepting the text im typing.