If you know the date and time of Backup and is\t is still in QHST Log you can do a DSPLOG put in the date and time parameters when backup occurred. It will show Libraries or objects saved, place cursor on that position and d\click F1 for help, should show VOLID at that point.
Good Practice to append your backup strategy to do a Printout of QHST during those time parameters to an Outque for archival purposes.
And if you don’t have the QHST information, all you can do is load the tape and run the DSPTAP command. That will give you information on what is on the tape.