Start out addressing a professor based upon degree or faculty rank.
A professor who has earned a doctorate degree should be called "Dr. (surname)."
A professor with a faculty rank of professor, assistant professor, or associate professor should be called "Professor (surname)". "Professor" without attaching the surname is also acceptable.
A visiting professor should be called "Professor (surname)" or "Dr. (surname)" if he or she holds a doctorate degree.
An adjunct or part-time instructor that does not hold a doctorate should be called "Mr. (surname)" or "Ms. (surname)."
Arrive at the first class on time. Most professors introduce themselves by writing their names and contact information on the board. Some professors verbally introduce themselves. Hearing a professor introduce himself will let you know exactly how he wants to be addressed.
Ask the professor how he or she prefers to be addressed if it is not made clear in the class introduction. Some professors who hold doctorates still prefer to be called "professor" while others are upset if they are not addressed as "doctor." There are even some professors who prefer going by their first name. It is best to not risk offending a professor by using the wrong term of address. Just ask.