title: Firebase, Flutter: Check if User Signed In With Apple
body: <p>Did some searching and couldn't find exactly what I was looking for. Is there a way to determine if a user signed in with Apple (or really any other provider) in Firebase with Flutter? See the image below. Firebase knows it was an Apple sign in, but not sure how to check the provider to see if was Apple or another method.</p>
  <p><a href="https://i.stack.imgur.com/47Mip.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/47Mip.png" alt="enter image description here" /></a></p>