Make a list and call for appointments. Five to ten is reasonable. This also gives you a chance to make judgments on the criteria mentioned below. As a student will be working with the professor for years. No matter how interesting the work, if you hate your professor, life will be hell in the lab.
Some people get along better than others.
You must feel comfortable talking and asking questions of your professor. While related to the above, this is clearly a separate consideration. Be sure your professor isn't too hands off or too hands on for your liking. Professors can be thought of as kings and queens of their labs and some act like it. Some people are unaffected by being the focus of fits of rage, others are quite bothered.