This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Beside the own backend module, EXT:solr provides a feature called “Search Index Inspector”. This Tool allows you to select a page or sysfolder and check what data is stored for this entity in the solr index.
The “Search Index Inspector” can be opened with the TYPO3 Info Module (“Web > Info > Search Index Inspector”) it shows the stored data in solr from the page or sysfolder that is selected in the pagetree: