Know DataFile's file#, and find out which backuppiece the datafile is in.

F:\oracle\product\10.2.0\db_1\bin>sqlplus/as sysdbasql*plus:release 10.2.0.4.0-production on Mon Feb 23 09:41:48 2  015Copyright (c) 1982, Oracle. All rights reserved.connected to:oracle Database 10g Enterprise Edition Release 10.2.0.4.0-64bit productionwith the part Itioning, OLAP, Data Mining and Real application testing options09:41:49 [email protected]> col handle format a400    9:41:59 [email protected]> Set Line 20009:42:13 [email protected]> Select d.file#, p.handle09:42:16 2 From V$backup_piece p, v$backup_datafile d09:42:16 3 where D.set_stamp = p.set_stamp09:42:17 4 and D.set_coun     t = p.set_count09:42:17 5 and d.file# = 2;         file# HANDLE--------------------------------------------------2 2 2 2 2         2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 H:\FSARCH\FULL_HOPVUMAD_1_120 rows Selected. Elapsed:00:00:00.0509:42:18 [email protected]>

Reference from:
How to Locate what Files is backed up and what RMAN Backup Set (document ID 246189.1)

Know DataFile's file#, and find out which backuppiece the datafile is in.