Try dropping in on a couple of faculty members and ask them. Try the younger ones, they don't know enough to mislead you. The secretaries will know who generates waiting lists and who generates a lot of dropped classes and incompletes. The graduate students will know. If there is a math club, its members will be dying to talk to you about this.
Some colleges publish student reviews of instructor performance.
Shell out a few bucks if necessary and take the time to help make the difference between enlightenment and boredom. In large universities you may have a choice of five or ten different classes in the same subject. In smaller places, hope that teaching is taken seriously. By asking around you should be able to find two or three likely candidates who do a great job. Try them out.