OK if using Backup to disk yes you do need to sort out your media sets, BUT you are also going to need separate Backup To Disk Devices. As even if you put a piece of media in a specific media set - unless it is also in a different device then Backup Exec can move it from one media set to another - look at the table regarding overwriting media in the Managing Media section of the Admin guide and you will see that every configuration option eventually includes "Overwrite Recyclable Media in any media set" and when tape choice is made using that option the tape will move to the media set of the job requesting it.
If you have separate B2D devices (with each job using the appropriate device) as we won't move a media between devices so it will stay in the media set used by jobs in that device.
Now to answer your actual question, how to identify what media to move is difficult and in fact almost impossible to do - reason is that if currently you are appending to the current BKF files and they were all in one media set then it is extremely likely that you have media that has data from more than one of the types of job you want to split up. You can find out which media was used by which job by reviewing job logs. You can also find out which data is on which media by using the approproate views in the restore selections - might be very time consuming sorting it out though - unless you were doing overwrite jobs and not appends anyway.
