title: Can persistence be checked to exist?
body: <p>Firebase enables a user login to persist. However, the user is not available on page load. The app must listen to <code>onAuthStateChanged</code> to know when the user is made available. The wait is short, but not insignificant.</p>
  
  <p>Is there any way to detect whether a user can be persisted <em>before</em> the user is made available? This would be helpful, to tailor the user experience suitably.</p>
  
  <p>Looking in <code>localStorage</code>, the data seems available, and so one would imagine that a simple method could return <code>true</code> or <code>false</code> whether any persistence data is available in <code>localStorage</code>. However, I could not find such a method in the documentation.</p>