Professional or other titles should whenever possible be ascertained and used. Many Europeans and Latin Americans attach great importance to titles such as Professor, Engineer, or Doctor (although the latter does not always refer to Doctor's of Medicine: in many cases it means simply the possession of a Bachelor's degree). They may use more than one of them ( e.g. "Herr Professor Ingenieur"  "Herr Doktor Doktor" ).There are some anomalies. For example, in Britain, surgeons practice a form of reverse snobbism and wish to be called "Mr. Jones". rather than "Doctor Jones" (who would be a Physician). But do the best you can and if in doubt, ask the persons concerned what is the correct way to address them. Generally, it is better to do this than to  blunder on in ignorance. 

However there are limits to this approach, particularly with high titles of rank or nobility.  It would not be appropriate, for example,  to ask the Queen of England how she should be addressed ( in fact "Your Majesty" on first meeting and " Ma'am", abbreviated from "Madame" after that).   Some things just have to be prepared in advance, and the Embassy of the country concerned will readily give advice, if the standard text books on Forms of Address (see Bibliography on our links page) cannot provide an answer. Or simply consult Webdiplomat.