Rule #1, ask the professor if you are unsure. Ask him what name he prefers to be called. I am a Chinese and a native Chinese speaker. I myself always like Westerners to ask me this question before they call my name.
In the example you provided, both
Yi can be used as English translation of Chinese surnames.
Zhang is a popular Chinese surname and
Yi happens to be one of my relative's surname. As you can see in this example, it's easy to make a mistake when calling Chinese names. Not to mention that the Chinese name convention is Surname first and Forename last (or Last name first and First name last).
If you cannot ask him for whatever reason and you have strong reason to believe that his surname is
Zhang, you should call him
Dr. Zhang. This would be the most appropriate way.