Start with the paper that came with the equipment. Pull the owner's manual, the installation sheet, and any maintenance page. Look for a heading such as maintenance, service, or routine care. It usually lists what gets done, how often, and sometimes what the technician should note. Write each interval in your own words beside the name of the system.
If the paper is gone, the nameplate on the unit gives you the model and serial number. With those in hand, you can find the maker's published maintenance guide, which is usually the same document the manual points to.
Turn the interval into a list you will read
Once you have the interval, count forward from the last logged visit. If there is no log, count from the install. Put each due point on a calendar you already check. A filter change that falls due several times a year belongs on the same calendar as the seasonal walk-through, not in a drawer with the manual.
Decide who owns each line
An interval with no name attached is only a wish. For each task, write one name: you, another member of the household, or a service company you have hired. Some tasks are yours to do with a cloth and a flashlight, such as clearing debris from around an outdoor unit. Others need a trained technician, such as a combustion check or any work on refrigerant. Mark which is which so the list does not stall at the first task nobody will touch.
Ask for the log, not just the visit
When a technician leaves, ask for a written note of what was checked, what was changed, and what they recommend before the next visit. Keep that note with the manual. It is the record that tells the next owner, and the next technician, where the system stands. A visit with no written note is hard to plan around later.
What to ask when you are buying a home
If you are looking at a house, ask the seller for the same paper: manuals, service notes, and any list of what was done and by whom. Compare what they show against the intervals you found. A gap in the record is not a verdict on the system. It is one more planning input, the same as the install year. The index treats age the same way, as a line on the calendar rather than a warning.
Where the index fits
The service-life guides on this site give typical planning ranges for each system. Use them to set the replacement window, and use the maker's interval to set the upkeep. If your main heating system is a gas furnace, start with the Gas Furnace page. If it is a heat pump, start with the Heat Pump page. For the seasonal visit, the fall furnace walk-through shows the order of checks.
If a service plan is part of the picture, read its terms for how it starts, how it ends, and whether it transfers to a new owner. Those terms are where the real questions sit. Any published figures belong in the index tables on this site, not in this guide.