No proof. And I venture to say we'll never have proof.

That's not to say that there is no ET out there, there almost certainly is. But intelligent life is so rare, and so few and far between that it's an almost negligible probability that we'd ever meet.

Even if we did meet, there'd be so little chance of proper communication between us. (Chimps and porpoises are very close to us in genetics and intelligence. But we still can't communicate very much at all.)

Looking at a night sky a long way from civilization, realizing that many of those "stars" were galaxies with as many "stars" as those that were visible to me then, and realizing that was reflexively true for those "stars" as well.

Green footprints painted onto the sidewalks of Roswell, NM. It was the most incontrovertible proof I've found of Entertaining the Tourists.