I'd ask a potential developer to give me URLs for a couple of projects he built. Anyone can claim he designed Google or Yahoo so I would also request that he include the name of the person he reported to at the site. Then you can contact that person by email or phone directly from their website (important, because if you email or phone any old address/number the developer gives you, how do you know it's someone actually associated with the site in question?). Ask about how it was working with the developer, if they feel satisfied with the results, etc.

You can also take a look at sites which you find interesting and well designed. Many times the developer will have a small link at the bottom of the main page so you can contact him. If not, send an email to the webmaster and inquire about who their developer is and follow up with the same inquiries as above about their work with that developer.

Kevin