If they were personal, they would have been in his papers at home (stored in the family basement/attic) or found when his office was cleaned out 20 years ago. Assuming of course, that they were genuine (ROFLOL). So they would have had to have been handed over by the family themselves, stolen from the family, or found 20 years ago and held in Al Capone's vault for safekeeping for just exactly this moment in time.
Is there anyone who could legitimately determine this?
No. They were "secret" documents "just" discovered. That's what makes them so mouth-wateringly tempting to the DemonRats. Killian is long dead, and since they were secret notes to himself, nobody else would have known about them -- thus requiring no other corroborating witnesses to attest to their legitimacy.
The 'Rats thought they had set up a hum-dinger of a trap (whom to rebut??), but it backfired in a matter of hours, thanks to (a) FReepers Buckhead, TankerKC, Howlin and others right here on FR who initially raised questions about the appearance of the documents even as the 60 Minutes show was on the air, (b) bloggers who linked to the FR posts and kept the issue buzzing, received numerous additional tips and professional input from real experts on their collective websites, and (c) AlGore, for inventing the internet.
To think, a few years ago this farce would have gone practically unchallenged, other than a few letters to the editor weeks later. The sheeple would have just raised eyebrows, accepted The Media's pronouncement as gospel truth, and that would have been the end of it.
Viva FR!