title: React: Detect a variable's PropTypes
body: <p>In a React component, if I declare:</p>
  
  <pre><code>MyComponent.propTypes = {
    // An object that could be one of many types
    header: PropTypes.oneOfType([
      PropTypes.string,
      PropTypes.number,
    ]),
  }
  </code></pre>
  
  <p>How do I know if <code>header</code> is a string or a number in my <code>render</code> method?</p>