Until a file has actually been pulled back out of it, a backup is a claim rather than a fact. Most of those claims hold up, broadly, with one exception nobody has gone looking for: the folder that was moved off the desktop two years ago, the second computer that was never enrolled, the phone that backs up photos but not the messages, the external drive that stopped mounting in April and has been reporting success to nobody ever since.
The test below takes an evening. It costs nothing beyond the time and it converts an assumption into a fact.
Step one: list what you would actually miss
Write it down before you look at any software. Photos going back to whenever they start. Tax records and receipts. The documents for the house. Anything creative you made. A password vault. Contacts. Text threads that matter to somebody.
Keep the list to what you would genuinely mourn rather than everything you own. Ten items is a normal answer. The list is the standard you are testing against, and writing it after you look at the backup software guarantees you will only test what the software already does.
Step two: restore one file from each item on the list
Not a check that the file appears in a listing. An actual restore, opened, on a device that is not the one the file lives on.
This is where most tests fail, and they fail in three predictable ways. The file restores but will not open, because the sync service kept a placeholder rather than the file. The file restores to a version from months ago, because that folder stopped syncing when a drive filled up. Or the file is not there at all, because the backup covers the user folder and this lives somewhere else.
Give yourself a stopwatch on the first one. How long it took to get a single file back is a good estimate of what a full recovery would feel like multiplied by several thousand.
Step three: find the second copy
A single copy in one place is storage, not backup. The arrangement worth having is simple to state: two copies on different devices, one of them somewhere else physically.
The somewhere-else copy is the one people skip, and it answers the failures that take everything at once. A house fire, a theft, a flood in the room where both the computer and the drive were sitting. A cloud account counts. So does a drive kept at a relative's house and swapped every few months, which costs nothing after the drive.
Step four: check what the backup does when the file is wrong rather than missing
Deleting a file is the failure people plan for. Corrupting one is the failure that happens. A document that saves badly, a photo library that gets confused, an accounting file that a program mangles on a bad shutdown, or a piece of malware that encrypts everything and then waits.
What protects you here is version history: the ability to reach back to how a file looked last Tuesday rather than how it looks now. Find out how far back yours goes. Thirty days is common on consumer services. If the answer turns out to be that yours keeps only the current state, that mirror is copying the damage as fast as it happens.
Step five: write the recovery down where someone else can find it
Two paragraphs on paper. Where the backups are, what account they sit under, how to get into that account, and who to call.
This is not a document for you. It is for the person who has to reconstruct a household's records while you are in the hospital, or for your executor. A backup that only one person knows how to open has a single point of failure and it is not the hardware. Put the paper somewhere a family member would look, and mention that it exists.
What it costs to keep running
Over five years the arithmetic is unremarkable. A consumer cloud plan runs a few dollars a month. An external drive lasts several years and costs less than a phone. Against that, reconstructing even part of what is on the list is expensive when it is possible at all, and the photographs are not on the list of things money reaches.
Keeping data out of the wrong hands and getting it back after a bad day are two different problems, and only the second one can be solved in advance. Large organizations are made to treat them separately. In the cybersecurity framework the National Institute of Standards and Technology wrote for them, recovery is a function in its own right, listed apart from protecting and detecting rather than folded into either. A household does not need the framework. It needs the evening.
Put a repeat of this evening in the calendar for a year out. The test takes twenty minutes the second time, because you already know where everything is, and it catches the drive that quietly stopped working in the meantime.