How do I check a tradesperson is legitimate?

Check the company at Companies House for filing history and how long it has actually traded. Verify any scheme membership at source — Gas Safe, NICEIC, NAPIT, FMB, NFRC — using the registration number rather than the logo. Ask for the public liability certificate and check the date and the limit. Read reviews for repeated patterns rather than the average score. Finally, ask to see completed work of the same type in person, which is the check most people skip and the one that tells you most.