I was asked this question a month or two ago.
How do you tell delusion from reality
Here is a definition (one of many) for delusion:
Delusion is rooted within the ego’s thinking we are separate from the universe and then to “project” a revised perception based only upon our own image.
So delusion is the active projection of ego, creating a false illusion of what reality is.
How to tell the difference between delusion and reality.
1) Use the third eye and cross-check the information at hand.
Interestingly when using the third eye: delusions have no connective substance.
2) Since delusions are powered by a person’s ego, they may seem real at first, yet they will lack the weight of reality when compared and poked at with all your senses and reason.
So you can determine a delusion when it is something that cannot be validated across multiple senses or axes of validation. Secondly, delusions will contain no valid extra information that allows a person to go outside their field of view. Since delusions are generated internally to a story, they contain no additional information that goes outside the story they are generated within.