Is it just me that recalls my dog the moment I hear an owner telling them to "be nice"? I've had it twice now where the dog has turned out to be distinctly snappy a few seconds later; there's no reason to tell a dog to "be nice" unless it has a history of being not nice.
My other rule of thumb (and my judge pants are firmly on for this one) is that the type of lead used tells you a lot about the owner and dog - extending leads typically for owners who lack knowledge, long lines for more knowledgeable owners of friendly dogs with crap recall, and short leads when used in a park where most dogs would be off lead for more knowledgeable owners of unfriendly dogs. I know I had an extending lead until I became more knowledgeable and realised how dangerous they can be!
Anyone else with judgey pants rules of thumb for dog life?