您可以通过User Credential检查该用户是新用户还是先前注册的用户，如果还不是该用户，则可以检索authResult.additionalUserInfo.isNewUser之前注册过.

You can check the user whether he is a new or previously registered user via User Credential and through you can retrieve the authResult.additionalUserInfo.isNewUser if the user is not already registered before.

您可以使用以下代码:

 Future<UserCredential> signInWithGoogle(BuildContext context) async {

    FirebaseAuth _auth = FirebaseAuth.instance;
    // Trigger the authentication flow
    final GoogleSignInAccount googleUser = await GoogleSignIn().signIn();

    // Obtain the auth details from the request
    final GoogleSignInAuthentication googleAuth =
        await googleUser.authentication;

    // Create a new credential
    final GoogleAuthCredential credential = GoogleAuthProvider.credential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );

    final UserCredential authResult =
        await _auth.signInWithCredential(credential);

    final User user = authResult.user;

    ///Her to check isNewUser OR Not
    if (authResult.additionalUserInfo.isNewUser) {
      if (user != null) {
      //You can her set data user in Fire store
      //Ex: Go to RegisterPage()
        }

      }
    } else {
     
      //Ex: Go to HomePage()
    }

    return await FirebaseAuth.instance.signInWithCredential(credential);
  }