Are you accredited?
Ask your builder if they are a member of a trade body to ensure you’re dealing with a legitimate company. Look for the Federation of Master Builders (FMB) or The National Federation of Builders (NFB).
These organisations have a code of practice and stringent membership criteria, including solvency and safety, so membership is a good indication you’re picking a reliable and trustworthy professional. Make sure you check that the builders’ membership is legitimate and current.
Can I see some completed projects?
While credentials are important, the proof is the pudding! Ask the builder to show you examples of completed projects, ideally similar to the one you’re planning. A quality builder will have plenty of case studies to show you. You can also ask the builder if they have any local projects in progress. Driving by for a look at a live building site can give you a good indication of how the company operates - such as how tidy they are.