What is "proof"? I leave fingerprints, I have my own DNA, I work a job, I live in a house, I pay my taxes, so, at least in the eyes of the good old US of A, I exist. I've also done my fair share of drugs, had several out of body experiences while hanging from hooks in my back and my chest, and have had visual and auditory hallucinations on several occasions, so don't take my word for it.
Honestly though, I think I exist if only for what I've experienced and endured. My body is almost completely covered in tattoos, scars, brands, piercings, implants, and modifications and I've felt and endured every one of them. I've suspended more times than I can count, and once while performing in a show I hung over 500lbs on 2 hooks in my back. I've sat through many many many hours of excruciating pain including having my tongue split and my penis filleted open (look up "subincision" or "meatotomy"). you'd think that if this existance was all a dream then I'd have woken up from it already.
Jason