It's possible you're in a coma and everything you're experiencing is a terribly detailed dream.
It's not very likely though.
It's not very likely though.
If you can't even remember things in the medium term that's a level beyond "trapped in a dream" that's much more hopeless.
So, with that assumption, you make a computer solve a problem that's impossible for a human to work through in a practical amount of time. And then you verify it got the right answer. This proves the math wasn't done by your dreaming brain. (At least it proves it to a pretty good certainty, and you can repeat the test.)
Many NP-complete problems are good candidates here for slow solutions and fast verifications.
Also for situations that have the standard dream level of fidelity, you can try basic reality checks like putting your finger through your hand or more conspicuously spinning along your own axis.
I thought this was just me! There have been times when I've "read" in dreams (signs, usually), but I don't actually visualize the words. I just "know" what they say.
I would even say "highly unlikely". I revisited how I understand "unlikely" after reading this:
> Radioactive capsule that fell off truck found in Australia... Radiation Services WA general manager Lauren Steen describing it as a "highly unlikely" scenario.