Ask about method, not price first
Ask what product they would use on your specific stone, how they handle unlacquered brass, and what they will not touch. A prepared company answers specifically and names its limits. An unprepared company answers that they clean everything.
Ask about consistency
Who arrives each visit? What happens when that person is unavailable? Is the scope written down, and who verifies it before the team leaves? These questions predict your experience in month eight far better than the first cleaning does.
Ask about discretion
Confirm the photography policy, social media policy, access handling, and whether confidentiality agreements are available. In a private residence, discretion is not a premium feature; it is a baseline requirement.