The dev team (possibly YP) did respond to something like this at some point awhile back. Pretty much they said 'it'll be clear who played pre-2.0' Upon which players were making jokes like 'guess we'll be covered in ash' etc. They didn't make it any more clear than this. Now this being said, there is plenty of stuff in game atm we can do that goes away after 2.0. Not only that, but lots of the items/lodestone history we have now will no longer be achieveable post 2.0.

1.Goobue
2.Purple Aytherite Quests
3.You won't start 2.0 as a rank 1 character
4. Possibly some primal weapons like Ifrit/Moogle
5. Possibly some of the dungeons like Darkhold
6. Your achievements/lodestone history
7. And well, you can see how long ANYONE has been playing based on their lodestone history.

Anyway that being said, it'll be BEYOND obvious who played pre-2.0 without any special rewards. I mean, really you don't think the new players will notice that your gear looks a little snazzier than their hempen doublets?