Updated June 10th, 2019.
If you use Time Machine for backups (and you should) it’s good to know when your Mac was most recently backed up. It’s easy to check: just click on the Time Machine icon in the menubar. You should see something like this:
The “Latest Backup” should be within 60 minutes of whenever you look (this picture indicates a problem– Time Machine hasn’t backed me up in a week!).
If you use Time Machine, but you don’t see it in the menubar, go to System Preferences/Time Machine and check the box to show Time Machine in the menu bar.