There is a nice tool in CMC that allows to check relations between objects. Something similar can be done in the query builder with some non documented features.
For instance, to find all reports that use a universe Finance, you can write:
SELECT * FROM ci_infoobjects, ci_appobjects, ci_systemobjects WHERE PARENTS("si_name='Webi-Universe'","si_name='Finance'")
PARENTS has two arguments. First is the relation type, second is the reference object.
If you want to find opposite – all universes used by a report, you can write
SELECT * FROM ci_infoobjects, ci_appobjects, ci_systemobjects WHERE CHILDREN("si_name='Webi-Universe'","si_name='Balance Sheet'")
What are the possible types of relations (first argument)? You can find the list querying CMS:
SELECT si_name FROM ci_systemobjects WHERE si_relation_table_name='RELATIONS'
Now you can also build such queries and export result to to Excel in InfoStore Query Builder: