As Andy has already mentioned, there is no better way to be 100% sure about a backup without doing a restore.
You can read the tape contents by using bpmedialist -mcontents command or through GUI
Media Contents Report
The Media Contents report lists the contents of media as read directly from the media. It lists the backup IDs that are on a single media ID. It does not list each individual file. This report does not apply to disk storage units. Note that the storage unit may stay in use for some time after the break if the following occurs: you attempt to abort the command by entering ctl-c and the requested media are not mounted or positioned. Each entry in the report appears as that area of the storage unit is read.
The -l format for the Media Contents report produces one line for each backup ID and contains the following fields. For more detail, see the Media Contents Report section in the NetBackup Administrator’s Guide. Any of the following fields that are not documented in that section are reserved for NetBackup internal use.
■ version (1 denotes a DB backup image, 2 denotes a regular backup image)
■ backup id
■ creation time
■ expiration time
■ retention level
■ fragment number
■ file number
■ block size (in bytes)
■ status
■ media_id
■ size
■ reserved
■ data_start
■ reserved
■ client_type *
■ copy_num *
■ sched_type *
■ flags *
■ opt_extra
■ mpx_headers
■ res1
■ policy name *
■ schedule label *
* These fields are significant only if version is 2.
Or you can configure an inline copy for a critical or on-demand backup, which will create multiple copy at the same using one read from client.