You can find information on your Nintex license in two ways.
Open the product's designer page, either through editing an existing workflow or creating a new one. Once the designer is open, go to Help > About and view the information there (see below, but please do not obfuscate the License ID info).
2. Through SharePoint Central Administration
First off, you'll require permissions to access Central Administration. In the Central Administration site, click on Nintex Workflow Management, and then Licensing.
Information on license ID, license type or expiry appears on this page.
In Nintex Workflow and Nintex Forms for Office 365 you may get asked for your Tenant ID. To find the tenant ID, open your Nintex Workflow or Forms designer, click on Help in the Ribbon, and then About.