<span class="application">Disks</span> lists all the current storage devices in the left pane of the window. When you select one of these devices, detailed information about it is displayed in the right pane: its partitions, its brand, its size, etc.
Find which device is your Tails USB stick. It should have 2 partitions: one labeled **Tails** and another one labeled **TailsData**, which corresponds to the Persistent Storage.