Nokia Lumia 521 - What is the Storage Check app on my Nokia Lumia with Windows phone 8? How can I get it?

It helps you visualize your phone storage consumption in more detail. You can also clean up temporary files using the Storage Check app.

You can get the storage check app via extras+info update. After the upgrade, the storage check app will be visible in the settings menu.

Note: Temporary file clearing deletes some of them, but not all. For example the Internet Explorer cache can be cleared from the application's own settings. Storage check doesn't do that as clearing the Internet Explorer cache removes cookies and login details too.