Many times we need to know which columns (in composite indexes) that make the index is function-based (BFI) instead of a normal index. If it is a simple index no mystery (index on only one column). In the case of multiple columns may be all, only one column or a few columns.
The more usual way, from what I’ve seen in my years of experience is to use the package DBMS_METADATA invoking the GET_DDL function, get the sql code and from there, make a study of it. The other less common way is by consulting the DBA_IND_EXPRESSIONS, as shown below;
SELECT COLUMN_POSITION, COLUMN_EXPRESSION -- Long type FROM DBA_IND_EXPRESSIONS WHERE INDEX_NAME = '<MY_BFI>' /
HTH – Antonio NAVARRO