If you are building a report and looking for the option (often found in formatting or shape menus):

A (also known as a product key or license key) is a unique alphanumeric string—typically 24 to 25 characters long, divided into five groups—that verifies your legal right to use the software. Unlike the old perpetual license model, Tableau has largely shifted to a subscription-based licensing model .

If you have a key and want to verify if it is valid (a "solid" key) or check its status, you can do this directly within Tableau Desktop:

Occurs when a key is used on more than the permitted number of devices (typically two for a single user). LBLM (Login-Based License Management):