You test your backups primarily to test your restore procedures so that when you're in crisis situation you'll know exactly what to do and when everybody will be panicking you'll be competent, confident, calm and will know exactly what to do and roughly how long the restore will take etc. etc. because by then restoring backups is a routine event.
The second thing you probably want to do is test data integrity, once you restored your critical data can production be resumed? Is nothing corrupted or incomplete?
You can and probably should test both of those things one small piece at a time. Only once you have the basics down should you attempt restoring a whole datacenter.
If you make backups of file systems and network shares for instance a suitable test would be to restore a specific directory at an alternate location and compare file-sizes, hashes and permissions with the original.
The next time you need to clone a database for testing, instead restore a production database from back-up.
Do a "bare-metal" OS restore on a VM if need be.
But backups and restores are just one aspect of a larger disaster recovery strategy and business continuity plan.
What will your business do when your current location would be lost due to natural disaster (fire, flooding, hurricane etc.)? Can it continue to operate from other existing locations, or is yours is the only location, will the business simply go bankrupt or will insurance money be used to rent emergency offices/containers?
That was the BCP strategy a couple of years ago at one company: a contract with HP, or maybe IBM at the time, to supply a datacenter in a container once a year for complete datacenter disaster recovery tests and having that on standby as well in case of acute disasters.
That company had 1 office facility and only tapes off-site (or maybe a tape-robot) and everything else in-house. The idea was that renting temporary furnished office space, getting internet connectivity and rerouting telephone numbers, getting desktops and printers etc. would be mostly commodity and easy to arrange. But IT slightly less so. The cost-benefit calculations for a twin-datacenter were unfavourable.
So initially every 6 months, but afterwards once a year, they did do a complete BCP test, but on temporary rented hardware: deploying VMWare, restoring the back-up server, restoring VM's with AD domain controllers, mail servers, database & application servers and file-shares.
A more contemporary BCP strategy could be cloud based and with both an off-premises backup copy online and you test your DR restore in the cloud as well, if you only need them a couple of days even a fairly large number of VM won't break the bank.