debug_auth.java

FirebaseUser user = mAuth.getCurrentUser(); // mAuth is your current firebase auth instance
user.getToken(true).addOnCompleteListener(this, new OnCompleteListener<GetTokenResult>() {
    @Override
    public void onComplete(@NonNull Task<GetTokenResult> task) {
        if (task.isSuccessful()) {
            Log.d(TAG, "token=" + task.getResult().getToken());
        } else {
            Log.e(TAG, "exception=" +task.getException().toString());
        }
    }
});