How could I make just a verify of data written on the tapes ?
You can run a verify job - http://www.symantec.com/docs/HOWTO74226
...is this data that has just been written to tape, or data currently contained on tape? If the latter (data contained on tape) then you're out of luck. The Verify job needed to have run as part of the job, or after it completed. There is nothing you can do once the job has completed.
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