Compare the promise with the work
Ask which tasks were included, which were performed, and whether any were skipped because of equipment type, access, or a discovered fault. Ask for any temperature readings, electrical measurements, or combustion findings that were part of the purchased scope. If there were no measurements, ask what the provider could reasonably conclude from the work performed.
For context, Carrier's example furnace-maintenance checklist calls for heating operation and supply and return temperature readings, along with component inspections. An actual visit may have a different agreed scope, but a full operating check should be explainable. Carrier example maintenance scope.
Do not assume every unit needs cleaning on every visit. 'Inspected and clean enough to leave alone' can be a valid finding. It is different from not inspecting. Likewise, an inspection does not imply that every part was disassembled or that a safety condition was ruled out by every possible method.
Ask for a useful follow-up
A direct message can help: 'I understood that this appointment included a furnace check. Please send the completed checklist, any readings, anything not checked, and what those findings mean for next steps.'
If the invoice and explanation do not match the advertised service, request a follow-up or a clear remedy under the provider's terms. Keep the discussion about the missing work rather than deciding that a particular number of minutes proves misconduct.
A second assessment may be sensible if a safety or performance concern remains unresolved. Share the first report so the next technician can distinguish an unperformed check from a confirmed defect. Do not accept replacement solely because someone says the prior visit was too short.
Your next action is to obtain the completed-task report. That record, compared with what you purchased and your actual furnace's needs, gives you a fair basis for deciding whether the visit delivered its value.