Found a nice little GUI for visualizing, assembling and debugging SharePoint 2013 Search API queries:
Use this tool to test out and debug search queries against the SharePoint 2013 Search REST API.
Learn how to build an HTTP POST query, and how the different parameters should be formatted.
After running the query, you can view all types of result sets returned; Primary Results, Refinement Results, Query Rules Results, Query Suggestions, in addition to the actual raw response received from the Search service.
Can be used to query both SharePoint 2013 On-Premise, and SharePoint Online.