How do I check data usage for my Pocket WiFi?

Our Pocket WiFi device has an in-built data counter so that you can check your data usage on its screen.
For instance, one of our devices has the screen like this:

Although the text is in Japanese, it's very intuitive and easy to understand for non-Japanese speakers.

You can also track your data usage on your end by using an app such as:

My Data Manager: (iOS, Android)
smartapp: (iOS, Android, Mac)