I never assume the first name is okay, although I'm pretty sure some of my classmates do. This term, a prof wrote only his first name on the board introducing himself. In this case, I call him that. Otherwise, I ask, "Do you prefer John or Dr. Smith," (ie whatever the actual names are), and I usually ask this in email. They always prefer first name at my school. Wasn't like that 25 years ago. No one used first name. Now, it's a more collegial atmosphere of learning, even though we are drippy undergraduates and not really their colleagues.