It is often difficult to know what to call someone, even when you know them. For example, if you meet Mr. Richard Smith in your work you might hear his PA calling him Mr. Smith while an older male colleague is calling him Smith. His friends at work may call him Richard, Dick or even Dicky.
So what do you call him? Well, you should probably start by calling him Mr. Smith. Hopefully, he will soon say, Please, call me Richard/Dick, and this should leave things clear. If you are unsure, you can always say, May I call you Richard?
Something extra could be said for a woman whose marital state you do not know with accuracy. In that case, call her Ms. Jennifer Whitaker. (Calling her Mrs. Jennifer Whitaker if she is not married could be very unfortunate for you!!).
Australians and Americans don't like to stand on ceremony and they will quickly move on to first name terms. This is true of younger British people as well. Typically, the Prime Minister has always encouraged everyone to call him Tony. And in Argentina there existed "someone" who urged everybody: Please, call me Carlos. But that is another story.
With a capital word NOT? I think NOT 
