In that situation, having some information about the instructors involved can help you decide which class to take. And who better to give you that information than students who have taken that class, or others, from the instructor you’re curious about?
Most schools have students fill out course evaluations at the end of the semester. These evaluations are your best source, if your school makes the information available. For instance, University of Pennsylvania has started posting them online. Other schools may publish them in other formats or make them available through individual academic departments or schools.
If you can’t find the school-sanctioned evaluations, look online. There are numerous sites in which people post ratings and comments on their instructors. RateMyProfessors.com is the biggest and best known of the bunch.