How does vision Hoid know he is in a vision

[Deception He/Him]

How does vision Hoid know he is in a vision? Every time that he is in one of Dalinar's SR visions, the vision version of him suddenly realizes he isn't real and gets all depressed. HOW?

[earthexile]

The Hoid who was present for those events would have already been informed and experienced enough to perceive the difference, it was thousands of years since the Shattering already. He may have experienced similar visions, and learned to discern when he was inside one, like the people in Inception. He also may have been carrying tools, when it really happened, that would help. He's usually got some weird stuff on him.

In the novel of "Project Hail Mary," the protagonist wakes up in a mysterious location, with no easy way of figuring out where he is or what's going on. But he's able to notice a subtly disturbing thing about the way an object falls when he drops it, because he's a science teacher who constantly reviews the means by which we investigate reality. Then he's able to slap together an experiment with the limited tools available, measure a few key numbers, and determines that the 'gravity' he's experiencing is indeed different from Earth standard.

So now without leaving a small windowless room, without anyone explaining anything, he knows he's not on his home planet. That's not the same as knowing exactly what's happening, but something isn't right. That's kind of how I figure it went with Vision Hoid.

[GG0z He/Him]

Pretty much, all visions are in the spiritual realm, meaning they are made of Investiture. Since Hoid has been through so much, he wasn’t only made of concentrated Investiture, which is supposed to copy people’s memories, he was also Cosmere-aware, which meant that the vision knew it was a vision. It was an overload of investiture that caused the vision Hoid to “glitch” and understand that he is a vision.

[Nitpicking]

We're all just speculating. I think the Exist dawnshard modified Hoid's soul, and also gave him so much Investiture that even his "Spiritual Shadow" in the Spiritual Realm has enough Investiture to become sapient the way Honor almost(?) has. It's not different (in my opinion) from the way the Blackthorn spren is now a sapient being, and it's like (but not the same) as the way the Stormfather created Sylphrena.

Once the Hoidspren was there and sapient, like GG0z said, it would have Hoid's knowledge of the Cosmere and realize what had happened. As Brandon wrote about in Snapshot, it isn't fun to suddenly realize you are only a simulation of a real being, and you'll shortly poof into nothingness.