Assumptions/preconditions:

The perception of [existence] is based solely on interactions between sensory input and [memory|memories], as interpreted in the [frontal lobe].
The determination of [reality] is not something we are capable of doing consciously (e.g. [dreams], [drugs]), except in certain circumstances. Furthermore, it is not a task for which we are well [evolution|prepared].
Without any basis for comparison, we are incapable of assessing how realistic, probable, or absurd our situation is.
The average number of times a person imagines a fictional interaction with another person is greater than one.

Then, think back: how many times have you imagined a (fictional or real) interaction between you and another person (or two third parties)? If not, do so. If the situation requires that the(se) character(s) remember things or receive sensory input, then give them these. Under the above conditions, the imaginary character would have no way of realizing their true state of existence, unless you, the imaginer, informs them.

Thus, there is a [probability] that you simply exist as an imaginary actor used to test a social situation. You may, in fact, be an actor in someone's thought experiment to test the above theory. In fact, the possibility that this imaginary actor could be made to recursively carry out this thought experiment greatly increases the probability that you are such an imaginary actor.

The question, then, is: how do you convince your imaginer to reveal the true state of your existence? Or, perhaps more importantly, how do you treat your own imaginary actors?

Are you a figment of an imagination? (idea)