How did Voldemort not realize Snape in the first book?

In the entire first book, obviously Snape is stopping Quirrel every chance he gets and is always saving Harry. How did Voldemort on the back of his head not ever mention or seem to notice this? Yes he was "playing the part of being Dumbledores man" but he easily could've turned a blind eye and let Harry die. How did Voldemort not realize right then and there he was a double spy or at least question why he did that?