Start with a simple test. List the systems in your house that heat, cool, wash, or power things. For each one, can you say the year it went in? If you can answer for all of them, you are in good shape. If you cannot, the plan you are building rests on guesses, and this is the first thing to fix.
How to tell you are planning from memory
You are working from memory when the install year is a guess based on when the house was built or when you moved in. You are working from memory when someone told you the unit was not that old and nobody wrote it down. You are also working from memory when the paperwork sits in a box nobody has opened, or with a previous owner, or when the labels on the equipment have been painted over.
Where to look
Start at the equipment. Most furnaces, boilers, water heaters, air handlers, and outdoor condensers carry a data plate with a serial number. Many makers encode the manufacture year in that serial number, and the maker's published decoding guide says how to read it. Look also for install years written in marker on a duct or pipe, service tags, and warranty cards left inside a cabinet door. An electrical panel usually shows a rated service size on its label, which is useful but is not an install year.
Next, check the house's records. Your local building department may hold permit history for work on heating, cooling, electrical, or plumbing systems at the address. Ask for that history. The records are not always complete, but they often give a year where nothing else does.
Then ask. Ask the seller, the previous owner, a neighbor who has been there a long time, or the service company that visited last. Write down exactly what each person says, and note how sure they sound.
What to do with the list
Once each system has a year, compare it with the typical service life on its page in this index. The gap between the install year and that typical service life gives you a planning window. It is not a prediction. Write the window next to the system, and put a time on your calendar to review it. Mark any system where the year is still a guess, and treat that line as unfinished work.
For a quick overview of how the major appliances compare, see the appliances ranked by typical service life page on this site.