Make sure the AC contractor can provide proof of a proper license. Ask for proof of insurance, to protect your home from damage.
Always ask the AC contractor to assess and explain what service will be provided.
Don’t be afraid to ask the AC contractor to specify what will be cleaned. As part of most air duct cleanings, the components include air ducts (supply and return), coils, drain, registers, and grills. Ask what chemicals will be used. Make certain that the AC contractor uses an agitation and vacuum collection machine while cleaning.