It's a bit awkward, but I often forget it...
Luckily, we can run either of these commands to find out. (Source)
cat /etc/*-releaselsb_release -ahostnamectl
Alternatively, we can use a _fetching tool like fastfetch
Which WindowManager
grep 'ExecStart=' /etc/systemd/system/display-manager.service
Desktop Manager:
printf 'Desktop: %s\nSession: %s\n' "$XDG_CURRENT_DESKTOP" "$GDMSESSION"