Ask the builder for references to get a list of those to contact about the builder's work, and then call each reference given. Ask for current projects and speak to those references as well, without the builder present. If possible, visit the site of the builder's current or past work with the owner's permission. This provides an eye-on look at the builder's work.
When speaking to references, it's important to ask questions about their experiences with the builder. Common inquiries include whether or not the work was done on-time, to the correct specifications, at the quoted price and if the person recommends and would use the builder again. Take notes on the responses references provide. For current local work, ask if it's possible to visit the site after completion of the work to check on the results.
Other questions to ask references include: Did the workers arrive on time and keep the site as clean as possible and did the builder keep the reference well-informed on the status of the work or any issues that came up? Keep in mind when researching references that builder-provided references are usually the showpieces of their work. Check with local contractor's guilds and inspectors to determine if there are any complaints against the builder as well.