Honestly, different people have different things that let them realize there is some form of afterlife. I have had several experiences with the supernatural. A few of which happened while I was an Atheist. My first experience could easily be explained away as trickery of perception. My second experience however defied explanation.
Those of you that live in Colorado Springs may be familiar with the Pikes Peak Community Center. I was performing a show there(with my band). I can't remember why they needed it, but they needed a ladder from the basement. I said, "Sure, why not? I'll go get it."
One of the people working there said to hold up and he would come with. and I responded with a little bravado that I could get it myself. It's only a ladder. He replied with, "I wouldn't go down there alone if I were you."
My immediate thought was, What is there a troll down there or what. But I proceeded down there myself. I got about 3/4 of the way down the hall and a sudden rush of fear hit me. I thought I heard a scream from a woman, and then things kinda got real dark. Now, keep in mind I didn't believe in ghosts, or anything of that nature. But what I saw and felt convinced me that something exists. It was a woman that screamed in my face and scratched my back. I felt it like it was a deep scratch across my back. When I let out a yell, the darkness went back to the lit hallway, and I was alone.
Now ordinarily I would have just chalked it up to losing my mind, or even gas buildup in the basement of the old building, but when my roadie came running down to check on me, he asked me what happened to my shirt. It had eight rips running from the center out, on my back.
I came up from the basement and the crew member looked at me and said, "You met her didn't ya."
Now I had not known this but apparently there was a nun that was killed(far more happened to her, but I won't go into details) down there back when the PPCC was the court house. That hallway used to connect the jailhouse to the courthouse.
Does that mean that the Christian God exists? No, but it was strong enough evidence for me that SOMETHING exists beyond death.
"I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it." Thomas Jefferson