Actual proof like seeing one. I'll believe in/acknowledge aliens if or when they're confirmed. I think it should be treated the same way we treat all other species or hypothetical species. If its existence is proven, we acknowledge it. If not, we don't. I don't want to believe things. I want to know things, especially when it comes to whether something exists or not. A species either exists or it doesn't. Aliens haven't been proven. Therefore, I don't acknowledge/believe in them. If they get confirmed, I'll acknowledge them.