Most states and cities don’t license air duct cleaners, so you’ll need to vet them some other way. Ask for proof of liability insurance and workers' compensation coverage.
Any reputable company will have this coverage and be happy to provide written proof. If they won’t do so, find another company — this one is probably unreliable, and you could be on the hook for big money if they damage your house or one of their employees is injured while on your property.
Make sure the company has been around a while and kept the same name. If a company has been around your community for years, they’re unlikely to disappear next week if you have a problem. If the air duct cleaning company popped up three weeks ago and doesn’t have an office location listed, treat them skeptically.
Ask for references, and check them. For a job that could cost as much as $1,000, it’s worth taking the time to get it right. Check out their reviews on Angie’s List as well.
Ask the company whether they follow EPA guidelines for air duct cleaning and follow through to make sure they do so.