How to Verify Time Machine Backups
Time Machine will verify the backup by comparing checksums, and it will alert the user if a problem or issue has been found. If the backup verifies fine, no issues will be reported. It’s possible the checksums will not match, indicating some sort of issue, corruption, or modification with the Time Machine backup, and Mac OS will offer instructions to attempt to correct the problem. It’s also possible the backup won’t have a valid checksum at all.
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While the Verify Time Machine backups feature has existed for a long time in Mac OS X and Mac OS, it’s important to note that only modern versions of Mac OS maintain a record of checksums associated with each backup snapshot, so if the backup was made prior to 10.12 or 10.11 it can not be verified by comparing the checksum this way.
I think this only detects damaged files, not missing files. The menu command is often disabled for me, for no apparent reason.
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Apparently this article only applies to Time Machine backups that had been done over the network. I use a directly-attached disk and found this long Ars thread very useful and informative: https://arstechnica.com/civis/viewtopic.php?f=19&t=1289851
"The menu command is often disabled for me, for no apparent reason."
The reason is because Time Machine is so impeccably reliable that it senses you have no rationale to do a verify.
