In order to check the stock unit of each item you should access your Back office -Items- Item list. Here you will see the number of stock of each item, in the card of which you have selected the function "Track inventory". If any item stock is low or it is not available at all, then next to the residue you will see a proper notification.
Additionally, you can use the filter "Stock level" in the top right corner of the screen. Just open it and select the option: "All items", "Low stock" or "Out of stock".