What a professional can check
For a gas furnace, useful questions include how ignition and heating operated, whether the venting and combustion-air arrangement were checked, and whether electrical and airflow-related findings were within the model's requirements. Combustion means burning fuel. Temperature rise means how much the furnace warms the air passing through it.
Carrier's example maintenance checklist includes operating temperature readings, burner and heat-exchanger work, blower inspection, electrical checks, and vent and gas-supply checks. It is an example of professional scope, not a promise that every advertised tune-up or MGI visit includes every task. Carrier maintenance checklist.
Ask whether the visit includes combustion measurements, what conditions trigger a deeper heat-exchanger inspection, and how findings will be reported. Electric furnaces and heat pumps need a different checklist from fuel-burning equipment. The technician should identify what you have before describing the work.
What belongs in homeowner care
Your owner manual may explain filter replacement, keeping accessible grilles clear, and noting changes in operation. Internal cleaning, electrical tests, gas adjustments, and protective-control tests need qualified work. For example, Trane's owner manual limits servicing to its stated owner tasks unless the person is qualified. Trane owner-care boundary.
Do not remove burner or blower panels to watch a test or hold down an access switch. A phone recording taken safely from outside the equipment, a symptom log, and the thermostat display can give the technician useful starting information.
Leave with something you can use
Ask for completed tasks, measured results with their allowed ranges, any areas not accessible, and findings ranked by urgency. A recommendation should say what problem the proposed work addresses. A general sticker saying 'checked' is less useful than a record tied to your furnace.
Maintenance cannot guarantee that nothing will fail later. Its value is in the actual care and information delivered. Your next step is to request a model-appropriate checklist and a written results summary before choosing the visit.