> What would convince you that the ability you experience is a delusion?
Hmm... That is an interesting question.
FWIW, there have been times when I've wondered, "Am I just insane?" but not for years now.
You would have to have some sort of record of most of my life, from say, age ten on. Especially the time I was homeless. (I dropped out of high school and lived on the street for about 4.5 years.) So many things happened during that time that would not be accepted as real by, uh, mainstream conventional viewpoints. "Mind reading" or "being psychic" was one of the more pedestrian aspects.
So you would have to have some sort of video recordings covering most of my life, and I'd have to go through them and see for myself how hundreds of "episodes" or events were just coincidences or hallucinations.
Interestingly enough, I was also suffering from depression during that time. However, that didn't seem to interfere with the, uh, esoteric stuff that was happening with and through me. As I said, mental illness and "psychic" phenomenon are orthogonal.
> I mean, some people are delusional, and they do experience a myriad of phenomena involving thought transmission, insertion, etc.
Oh to be sure! Schizophrenia is a hellofa thing. I remember reading a book written by someone who became schizophrenic and then healed from it, and they e.g. thought that all bus drivers formed a sinister psychic cabal that ruled the world!
I hasten to point out that you don't have to be crazy to fall into the trap of erroneously believing that you can "read minds", although we seldom call it that. It's a pretty common hallucination that we know what the other person is thinking, eh?
> So what is it that tells you it's real and fundamentally different in quality?
I was raised in a pro-science way (although I'm not a scientist myself) and some of my personal heroes include noted skeptics like Carl Sagan and Isaac Asimov. So as I began to investigate the wider world beyond convention I did my best to test these things.
I should point out that, although I can (or believe I can) see thought forms that are not my own (and really, as I said above no thoughts are properly "our own") I didn't try to read other people's minds. It seemed voyeuristic, y'know? Also, for my own weirdo reasons, I wasn't really interested in what other people were thinking, I was driven to find the truth about reality and most folks are just "watching TV" in their heads.
Anyway, there were just a lot of confirmations, over and over again. To the point where it was almost ridiculous. Getting back to the original question (about developing intuition) I was living in a stream of "coincidences" that happened all day every day. For example, for about two years, I never hit a red light. What I mean is that every time I came to an intersection the light was green in my direction. I was walking a lot back then, all day, all over the city, green light green light green light.
As a counter-example (not of my personal experience, but of fraudulent "psychics") I once went to visit a friend of mine and saw the sign of a local psychic shop in his stairwell. Asked why it was there he told me, "I was drunk and I figured that if they really were psychic they would have known I was gonna steal it and they wouldn't have left it out." Impeccable reasoning, if a bit mean.
My deal was the opposite of that. Everything fell into place, over and over again. I could go to a city I'd never been to before, walk straight to a "random" bus stop, board the first bus that came by, and arrive at my destination. No map, no schedule, just "following my feet." Call it intuition.
> Being able to see thoughts of someone who is operating a combination lock might be handy.
What? To steal from them? That's criminal. What do you want from knowing that combo? Money? What do you want that money for? Food? Just get food. :)