I regularly have conversations with a colleague who is also a pastor. In our most recent sit down he asked me near the end, "What evidence would you need to prove that god exists?", to which I replied, "A book not written by man." And so I meant not inspired or revealed by some god, it certainly stopped his line of questioning immediately because no one has ever brought this to his attention.
Yeah, I've given this a fair amount of thought, and never come up with anything really concrete. Almost anything I can come up with has at least two other equally likely possibilities. Hyper advanced aliens messing with us, or we are living inside a simulation. Either is as likely as a god, and from our perspective how could we tell?