Jeff Carlson, TidBITS:

You may not even have noticed that your Mac is now running APFS. To find out, open Disk Utility, click your startup disk in the sidebar, and look under the disk name.

As Jeff suggests, take a minute to launch Disk Utility, click on your startup disk and look at the text underneath the volume name. Mine says “APFS Volume • APFS (Encrypted)”.

This is a terrific read, chock full of detail on APFS, but very readable. And the fact that the transition to APFS has been so seamless for so many says a lot about the APFS engineering team.