I agree, the Better Business Bureau is a great place to start. They give grade A-F rankings for all businesses in their listing. Another place to check is complaints.com, complaintsboard.com and ripoffreport.com.
You can read complaints, often with specific details from former customers. Take it with a grain of salt as you never know if someone is venting, likes to complain or if it's a fake complaint from a competitor. Even good companies will receive complaints sometimes. That said, if you find a company that you're not sure about and they have dozens of well articulated complaints, you may not want to do business with them.