Showing related entries field
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 ) %}