1. Call the individual or institution holding the course. Determine the credentials of the instructor, and ask for references.
2. Phone the local police station, and ask for a reference. The chances are good which they know of all self defense courses in your area, plus they probably also understand which one is the best. They may even offer their own self-defense courses.
3. Ask whether there's a sample class, or if you would pay for just one session, as opposed to paying for multiple sessions.