Taking the piss.

"How do you like it?"
I gazed along the invisible trail the voice had left through the cold November air. It led to a bench by a bus stop, and belonged to a man in his twenties. He was looking a bit rough, a couple of days growth on his chin and a tattered hat on his head.
"How do I like what" I asked, looking in to his hollow eyes.
"Being dead, of course."
"I'm not dead" I said, my left foot creating semi-circular patterns in the snow.
"Of course you are, dear" he said and lit a cigarette.
"No I'm not."
"How do you know?" The smoke from his cigarette travelled towards the sky.
"Well I... I... I don't know."

How do you actually know you're not dead?