here's what would convince me:
Everyone on earth is elevated onto space. Despite the lack of oxygen, we can all breath and don't feel cold. A 'voice' tells us that this is the creator of everything and everyone. Amongst other stuff it tells us things that only we know about ourselves. When we are finally put back down to earth, we each find in our home a book / cd / hard drive / whatever with a recorded transcript of what we've just been told, in our own language and dialect. This recorded medium is indestructible and unalterable. We can verify that everyone on the planet shared the same experience.
That would be all.
"In life, you have to face that some days you'll be the pigeon and some days you'll be the statue."