As seen in greater (coding) details in this lab, the communication between the Android client and the AppEngine backend is powered by Google Cloud Endpoints. Beyond offering an easy way for the developer to expose server-side business logic as RESTful endpoints, this technology offers the ability to use the Google APIs Explorer, that same tool used to explore any of the Google APIs (Maps, Calendar, Storage, Drive, etc.), only this time for any of your APIs.
To explore the APIs exposed by your backend, simply point your browser to
http://<project_id>.appspot.com/_ah/api/explorer
Here's the API Explorer documentation page: https://developers.google.com/explorer-help/