The structure will depend on the type of secondary index.
ie: USI/NPPI , NUSI/PPI, etc
Check out Database Design documentation for your release.
If you show the base table, you will see the columns of the secondary indexes.
If you don't have permissions to access the base table, ask the DBA to show the table and you will have the SI's columns.
Assume you don't have select access on DBC.