Showing related entries field I have a section called Services. I have another section called Case Studies. 

For each Case Study I want to show which service is being used so I add an entry field and choose the right service. On my case study I can see that the service is showing as it should.

On the Service page I want to see which case studies are being associated with the Service.
So your Case Study has a field called services which is an entry relation. You show the services on that Case Study page using 

// entry is the current Case Study
{% set related_services = craft.entries.section( 'services' ).relatedTo( entry ) %}


To show all Case Studies that are related to a Service you can use the same entries query. 

// entry is the current Service
{% set related_case_studies = craft.entries.section( 'caseStudies' ).relatedTo( entry ) %}