Notes from the Google Webmaster Structured Data Hangout on the 13th of December, 2016.
Use Site: to Test Rich Snippets Against Policy and Quality Criteria
The Rich Snippet testing tool only checks if the syntax is valid. Use Site: to check how your rich snippets will be displayed and see if they conform to the policy requirements and meet the quality criteria.
Apply Product and Aggregate Review Markup to Dedicated Product Pages
Product markup and aggregate reviews markup should be applied to pages dedicated to a single item, and should not be applied to generic pages which include multiple items, like category pages.
Marked Up Reviews Must be Visible on the Page
Review content which is marked up, must be visible on the page with the markup. You should not use the reviews markup if you don’t actually display the review. If they are coming from a 3rd party site, you should link to the original source.
The Structured Data Testing Tool Reports Warnings as Errors
The Structured Data testing tool reports some issues such as incomplete markup as errors instead of warnings which can be misleading.
The Sitelinks Search box markup is only used when Google is already showing the Sitelinks search box, to direct Google to a specific search URL. Without the markup, Google will do a site: query in Google’s search results.