In High Sierra you can easily determine if a volume is formatted as APFS or HFS+ (Mac OS Extended) by using the Finder to highlight the volume name, then click on File->Get Info, and see what’s listed as the Format.

Determining the format type is an important High Sierra troubleshooting step, because depending on the hardware configuration some Macs running 10.13 will be booted from APFS and other will be booted from HFS+.  These file systems are very different under-the-hood, and apps may exhibit problems with one but not the other.  For example, the automatic backup option in Quicken 2007 is reported to fail with APFS, but works with HFS+.