And for the very obvious you will usually find Powered by PHP Link Directory or Powered by PhpLD in the footer.
I think the 'admin' method is one of the best unless someone has purposefully renamed it. Because you can get powered by removed after some payment.
Best method is directory.com/admin IMO. Because you can't see footer link there if directory owner paid full $75 for phpLD license without link back DON.
why you need to know that? herm... i think better you go your own business, and do it what you want... sorry this what i think....