But seriously, that Pentagon disclosure last year on the AATIP that flew over everyone's head (no pun intended) was enough evidence for me. I wouldn't classify it as definitive "proof", but the evidence was pretty strong. When the United States government admits it's been tracking technology that they can't explain and that they've never encountered from any other military aircraft on Earth, you can pretty much conclude that intelligent alien life exists.
This is separate from the reality that if you even have a basic understanding of how astronomically, mind-bendingly, insanely laughably large the universe is, you'd already realize the probability of humans being the only intelligent species in the cosmos is just about zero.