I am presenting my research work at a conference. In my poster, I am crediting my research advisor. For anonymity purposes, say his name is John Smith. He is a PhD so he is often addressed Dr. Smith but he is a professor at my university so his students call him Prof. Smith. In my poster, should I credit him as Dr. John Smith or Prof. John Smith?
Short answer: Use "Professor".
Why? Because that is your adviser's official title at your university. It's a recognised "rank", and so he should be addressed as such, especially if you are introducing him in the capacity of your work at your university.
Best answer: Ask them how they would like to be referred to.
"Absolute is the right of any man to spell his name 'Jones' and have it pronounced 'Smith'."
If unable to do that, I'd use whatever form they last published under, which they presumably found acceptable. Or contact their department and ask if anyone there knows.
Another Short Answer: Use "Dr." or "..., PhD."
Acknowledging the PhD is the safest route if your concern is avoiding his being offended.
If a professor holds a doctorate degree, he or she will not be offended if you acknowledge it with the "..., PhD." suffix. However, if you do not acknowledge it, there may be some who would be offended, especially given that you're working in an academic setting, and there is some level of prestige among their peers involved in your work. So, it's safer to acknowledge the PhD.
If the professor does not hold the degree, then "Professor" is the right choice, obviously.
In my experience at many academic conferences, titles are never used. I've never seen anyone credited as "Dr Smith", "Professor Smith" or anything else involving a title. Just credit them as "John Smith". (This is in theoretical computer science; it's possible that other fields differ.)
I don't know for sure what the reason for this is but I would speculate as follows. First, almost everybody at an academic conference either has a PhD or is studying for one. When you're in a room with 50 PhDs and 20 PhD students (and maybe that one guy from industry who doesn't have a PhD), there's not a lot of point drawing attention to who has a PhD and who doesn't. Second, although one naturally bases opinions on all kinds of things, acknowledging somebody as "Professor Smith" sounds like an appeal to authority: Smith's work should be judged on its intrinsic quality, not on Smith's position on the career ladder. Third, different countries use titles in different ways. In the US, essentially any permanent member of the academic staff is a professor; in the UK, a professorship corresponds roughly to a named chair in the US, i.e., the title applies only to a relatively small number of very senior academics.