how do I know a subvolume is a snapshot?

[Ulli Horlacher <framstag@rus.uni-stuttgart.de>]

I thought, I can recognize a snapshot when it has a Parent UUID, but this
is not true for snapshots of toplevel subvolumes: 

root@trulla:/# btrfs version
btrfs-progs v4.5.3+20160729

root@trulla:/# btrfs subvolume show /mnt/tmp
/mnt/tmp is toplevel subvolume

root@trulla:/# btrfs subvolume snapshot /mnt/tmp /mnt/tmp/ss
Create a snapshot of '/mnt/tmp' in '/mnt/tmp/ss'

root@trulla:/# btrfs subvolume create /mnt/tmp/xx
Create subvolume '/mnt/tmp/xx'

root@trulla:/# btrfs subvolume show /mnt/tmp/ss
/mnt/tmp/ss
        Name:                   ss
        UUID:                   7732bdde-0485-204e-b41b-833376e791da
        Parent UUID:            -
        Received UUID:          -
        Creation time:          2019-07-17 01:02:48 +0200
        Subvolume ID:           270
        Generation:             60
        Gen at creation:        60
        Parent ID:              5
        Top level ID:           5
        Flags:                  -
        Snapshot(s):

root@trulla:/# btrfs subvolume show /mnt/tmp/xx
/mnt/tmp/xx
        Name:                   xx
        UUID:                   342b2065-1679-8245-bd76-8da598cc33d8
        Parent UUID:            -
        Received UUID:          -
        Creation time:          2019-07-17 01:03:02 +0200
        Subvolume ID:           271
        Generation:             61
        Gen at creation:        61
        Parent ID:              5
        Top level ID:           5
        Flags:                  -
        Snapshot(s):

How do I know that /mnt/tmp/ss is a snapshot?
I cannot see a snapshot identifier.

-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum TIK         
Universitaet Stuttgart         E-Mail: horlacher@tik.uni-stuttgart.de
Allmandring 30a                Tel:    ++49-711-68565868
70569 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/
REF:<20190716232456.GA26411@tik.uni-stuttgart.de>