How do I verify that current FirebaseUser exists?

You can use the reload method of FirebaseUser.

final FirebaseUser firebaseUser = await FirebaseAuth.instance.currentUser();
await firebaseUser.reload();

// This should print `true` if the user is deleted in the Firebase Console.
print(await FirebaseAuth.instance.currentUser() == null);

If your user is deleted in the Firebase Console (or disabled) and you call reload on the user in Flutter, the next time you call currentUser, it should return null, i.e. there is no user authenticated on this device.

using angular,

isLoggedIn() {
   return this.afAuth.authState.pipe(first())
}

doSomething() {
  isLoggedIn().pipe(
    tap(user => {
      if (user) {
        // do something
      } else {
        // do something else
      }
    })
  )
  .subscribe()
}