When I read topics like this I so wish that I had videoed Miss Sadi having one of her nightly "Grumble sessions" with DH, the body language said she was about to rip him to pieces....she never did, it was just fun.
What I am trying to say is that although many of the replies have given helpful clues as to the difference between smiling and snarling, but the only way you really know is if you know your dog/s.....this is obviously tricky to gauge if your dog is new to you, so I would just say always be careful, but don't let it stop you interacting with your dog, mostly they mean us no harm at all but as we don't speak the same language sometimes things get lost in translation. Follow your gut feeling and don't be too upset if sometimes there are misunderstandings.