More than once it's crossed my mind that I have personally encountered a very very very small slice of the real world, and anything outside of that observed on the internet could very well be a fabrication.

I base my belief in what I see online on cross-checking, general trust-worthiness of sources I encounter, and my gut feeling for the difficulty of faking something, but there's nothing stopping a determined adversary from inserting things into my perceived reality.

A sufficiently advanced AI could be generating the internet on the fly just for me and I wouldn't be any the wiser.