Oracle 12c: textual search over all PL/SQL code
Provided that the following is true:

1) The function is NOT in a package
2) The function call is NOT dynamic SQL

Then you can simply see what its dependencies are from dba_dependencies:

SELECT * 
  FROM dba_dependencies 
 WHERE referenced_owner = 'MYOWNER' AND referenced_name = 'MYOBJECT';


Otherwise, you can check dba_source:

SELECT * 
  FROM dba_source 
 WHERE upper(text) like '%MYOWNER.MYOBJECT%';


To view the source for all objects for a schema:

SELECT * 
  FROM dba_source 
 WHERE owner = 'MYOWNER'
 ORDER BY type, name, line;