check if a comment was made by a registered user

$comment = get_comment( $CommentID );
$commentatorID = $comment->user_id;
  if ($commentatorID > 0 ) {
     //Comment made by registered user, do stuff
  }

From the codex get_comment(), about the return value of user_id:

user_id
(integer) The comment author’s ID if he is registered (0 otherwise)