In any professional setting, the place (ie elementary vs university, banks vs hospitals, etc) and the circumstance don't matter.

If you want to address any person the way they want to be addressed (which is the only way should you ever address someone), you do as CarolC said. Which is to ask them "How do you wish to/prefer to be addressed?"

It really is that simple, and not anything to worry about. I'm in my late 40s and often run into younger colleagues who seem clueless about this most basic, simple piece of etiquette. Why is that? I mean, it isn't like I learned these things at charm school.

And I wouldn't recommend Allan C's method. It puts the focus of attention on himself, not the person being addressed, which is a great faux pas in important social settings, particularly academic ones.

And by important, I mean important to you GUEST Helen. You said it was important to you to do this right and make a good impression. So do just as CarolC suggested, and it will put both you and the other person immediately at ease, as it shows you *do* respect the person enough to ask.