User-review websites are a great starting point. If enough people comment about a particular store or vendor, it's easy to see a trend in service and quality. While I will contact the vendors on my list, and ask for references, this entails minimal effort!
Laura O. on yelp reviewed all of the services/vendors she used for her wedding. Not only do I get her opinion, I also gain greater exposure to other vendors I may not have known about.
On Wedding Mapper, people can give their reviews as well (not all are wedding-related reviews). [Find reviews on reception sites here.]