It's looking incredibly likely that Square Enix will drop a demo for Final Fantasy 7 Rebirth next week. There are multiple factors to this educated guesswork at the time of writing: Square Enix loves releasing demos even for its biggest games, Rebirth is getting its own State of Play on the 6th February, and now, the ever-reliable PlayStation Game Size says there's evidence of the supposed demo on Sony's servers.
The informative Twitter account also points out that players will receive 'Kupo Charm - Survival Set' for giving the demo a shot, and it's possible that there'll be some kind of save transfer. We don't know if this means that the demo will cover the game's prologue and you'll be able to carry your progress over to the full release, or whether it's just highlighting the ability to unlock in-game rewards.