Very old furnace- I need a technician to inspect it-What would be a good check up list?
No warranty is generally provided. A record of inspection is provided, with notes about what was checked and the readings that were recorded at the end of the adjustment procedure.

If you have been (as it would appear) in the habit of adjusting your furnace yourself, the fact that a licensed and insured service technician has looked at it in no way protects you from anything you have done to it, or may do to it again after the service person leaves.