How to tell delusions from reality?

Are the grotesque and hideous figures or objects seen by men in delirium-tremens real or imaginary?

Those who drink wine to the extent of stealing away reason or destroying the proper action of that organ of the brain by which the spirit works are subject to all the consequences of the wrong done to the organ. They see the frightful scenes and figures and imagine them to be realities—they are, however, only the effects arising from a deranged and outraged brain. There is nothing real in anything seen by the poor drunkard while in this condition. This can be easily tested by bringing one who is gifted as a seer into the presence of a person thus affected. You will find that the seer will be unable to perceive anything, but that which is natural.

—Hafed, Prince of Persia