Connect to the RMAN
$ rman target=/
Once in the RMAN, create special channel to Query the TAPE
RMAN> allocate channel for maintenance type sbt_tape;
Find the backups available on the TAPE
RMAN> list backup of database summary;
The output from the above command will show the summary of all the backups available. It may give you the backups on the Disk as well. Just see those with sbt_tape in the Device Type column.