Found this super helpful when trying to see if a field is even getting indexed at all, and if so what the values are so you can test your query logic with actual values in the index.

The same thing can be achieved in the Solr Admin by setting rows = 0, check the facet box, and put the field you want to know the values of in the facet.field textbox.

yoursolrurl:8983/solr/your_index/select?q=*%3A*&rows=0&wt=json&indent=true&facet=true&facet.field=field_you_want_to_know

Example below will return all template names and associated counts of index items with that value:

http://yoursolrurl:8983/solr/sitecore_master_index/select?q=*%3A*&rows=0&wt=json&indent=true&facet=true&facet.field=_templatename