How to confirm if FileVault encryption has fully completed
APFS volumes
It seems that the only way to tell for APFS volumes is to use fdesetup status. This fantastic blog offers a script which basically uses that command to determine the status. In the article it reads:

If encrypted, the following message is displayed:
FileVault is On.
If not encrypted, the following message is displayed:
FileVault is Off.

These are basically the outputs of fdesetup status.
Non-APFS volumes
For non-APFS volumes there is more information. Try diskutil cs list or diskutil cs list | grep &quot;Conversion Status&quot;, respectively and look for Conversion Status. See also the accepted answer here.
$ diskutil cs list

CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group XXX-XXX-XXX-XXX
    =========================================================
    Name:         VolumeXY
    Status:       Online
    Size:         196521975808 B (196.5 GB)
    Free Space:   0 B (0 B)
    |
    +-&lt; Physical Volume XXX-XXX-XXX-XXX
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk2s2
    |   Status:   Online
    |   Size:     196521975808 B (196.5 GB)
    |
    +-&gt; Logical Volume Family XXX-XXX-XXX-XXX
        ----------------------------------------------------------
        Encryption Type:         AES-XTS
        Encryption Status:       Locked
        Conversion Status:       Complete
        High Level Queries:      Fully Secure
        |                        Passphrase Required
        |                        Accepts New Users
        |                        Has Visible Users
        |                        Has Volume Key
        |
        +-&gt; Logical Volume XXX-XXX-XXX-XXX
            ---------------------------------------------------
            Disk:                  -none-
            Status:                Locked
            Size (Total):          196169654272 B (196.2 GB)
            Revertible:            Yes (unlock and decryption required)
            LV Name:               SD
            Content Hint:          Apple_HFS