If bitcoin is still in circulation, then proving is easy. If it is a relic, and something of interesting collectible nature, then it would be like proving your tablet IOU from the BC times is legit.

But in any case, most users have a hard time understanding cryptocurrency systems. Thus, it would be very easy to provide bogus info which looks convincing to a layperson. And because fakes would be common, it would take extra care to _really_ prove your collection is real.