When workflow actions are enabled in SharePoint, the web.config files for the web applications are updated with "safe controls."
To confirm that web.config files were updated with Nintex "safe controls"
From IIS Manager, right-click the web application, select Explore to view its files in Windows Explorer, and then open the web.config file in a text editor.
Example excerpt from web.config file indicating enabled actions:
<SafeControl Assembly="Nintex.Workflow.ServerControls, Version=1.0.0.0, Culture=neutral, PublicKeyToken=913f6bae0ca5ae12" Namespace="Nintex.Workflow.ServerControls" TypeName="*" Safe="True" />