All one needs are bone fide miracles, events that simply cannot be explained through natural laws (including the potential for human error).
Easier said than done. If all the stories I've heard from people are true, then it seems pretty conclusively proven to me that bona fide miracles have happened. However, how are you going to prove such a thing. A person said that a couple helped him with his car on the road, diagnosed his small daughter with appendicitis, and then directed him to a hospital. The couple turned out to be the founders of the hospital, long since dead.
However, the guy could have been lying. He could have been hallucinating. He could have been wrong in recognizing the couple as the ones who founded the hospital.
A doctor had a heart attack and wrecked his car. A truck driver found him on the deserted side road, saying he'd gotten a CB call from the doctor. However, the doctor had no CB and had sent no call. Did it really happen? We have only the doctor's word.
I prayed for my nephew's eyes in response to a letter from my sister. He had an infection that had taken all the sight from one eye and 50% from the other (so said her letter). When I prayed, I told the folks with me that I felt like something happened, but I couldn't get everything out when I prayed. It felt like something was undone. The next letter was only a week later. My nephew had only a small infection left, and all his eyesight had returned. The infection had been increasing for six months, and my sister's letter had seemed hopeless. Did my prayers have anything to do with the recovery? Obviously, I'm going to think so, but how can anyone prove it.
Bona fide miracles, even when they do happen (assuming they really did) are unlikely to lend themselves easily to proof, because they are not repeatable.