If you are looking for to hire a React Developer you can evaluate these items:

• High-level proficiency of JavaScript and ES6 (Destructuring, Promises, Object Literals, asynchronous methods).
• Understanding of Redux and Hooks for state management
• Understanding of Node.js and NPM for third-party integrations
• Can set up a database and interact with its APIs
• Has knowledge of MVC design pattern
• React-Router
• Use of a State Management Tool like Redux (Actions, Action creators, reducers, Sagas).
• Testing through a tool like Jest and Enzyme.
• UI Components library (The most popular is Material-UI).
• Deployment in service like Vercel or Digital Ocean.

As you can see, there are many things that a React developer needs to know to be a senior developer, and the majority of these skills are learned by developers after 3 years. Even, in IT Staff augmentation companies it's very common to hire only developers with this experience.