Interesting question... Let's put aside opinions and try to list the facts:
The preferred for of addressing a prof, much like anyone else, will vary immensely. It ultimately depends on many factors like:
- culture in that country ...
- and in the country of origin, if the prof in question has a different background
- culture in that particular faculty/department (usually less formality/honourifics in Maths/CS/Physics etc)
- how down-to-earth that particular person is
If you don't know something regarding another person, you essentially need to acquire that piece of information. It won't magically dawn on you... So your options are either to ask that person, or to ask someone else that knows this person.
The risk is, if you ask another person, you might be making the assumption that the person you ask has a different level of relationship with the prof. It might be so that the grad student you ask "goes way back" with the prof, and while they might be on a first name basis, you might not have the same grounds to stand on.
Bottomline: If you don't ask you can't know the right way to address the person. Noone can really blame you for asking how to refer to a person, especially if they are at a higher position than you, implicitly or explicitly.
You might be considered unnecessarily formal, or stiff, but still it's better than making an assumption and making a rude mistake, especially if the person in question comes from a culture where titles are taken very seriously.