Verify your backup disk on Mac
If you back up to a network disk, Time Machine periodically verifies that your backups are in good condition. This scheduled verification happens automatically, but you can manually verify a backup at any time.
On your Mac, click the Time Machine icon
in the menu bar. Press and hold the Option key, then choose Verify Backups.
If the Time Machine icon isn’t in the menu bar, choose Apple menu 
> System Preferences, click Time Machine, then select “Show Time Machine in menu bar.”