This is actually a version of an old con that has been performed for centuries. It works great for Halloween because of the association with the super natural and this activity(if it works) will wow your guests and add some magic to your party.
There are three roles to be played in this Halloween prank; the salesman, the skeptic and the mystic. All three have important jobs to perform and if any fail the prank will not work. So choose your partners wisely. It all starts with…
The Salesman: At some point in party the person playing salesman will casually mention in conversation that the person playing the mystic has some psychic ability. Hopefully a few guests not involved with the prank will express interest. Once there is some discussion it is time for the skeptic to get involved and the role of the salesman is over. From this point on the salesman should quietly let events take place without much comment. The more the salesman is evolved after initially mentioning the psychic ability the more people will be suspicious of a set up. It now depends on…
The Skeptic: Once the conversation about the psychic ability has started the skeptic should become very interested. With any luck, one of the other guests will suggest that a test be performed at the party. If this happens the skeptic should immediately step forward and offer to run the test. If no one else has the idea the skeptic should suggest that they test…
The Mystic: The person playing the mystic from the outset should be very humble and noncommittal about their psychic ability. The story is that they have been able to show psychic ability in the past, but it does not always work and they have no idea how or why they are sometimes able to sense information they should not know. The mystic should appear almost embarrassed by the attention their psychic ability is getting from the other guests. Once the test is offered by the skeptic the mystic will halfheartedly agree, since people seem so interested.
The Prank: The skeptic and the mystic should trade ideas as to how the test will work. Eventually they settle on the mystic standing in another room with the door shut and the skeptic will call out five items which are in the room where the skeptic and all the Halloween party guests are located. The skeptic will be touching one of the five items when they call out the list and all of the party guests will be focusing on psychically projecting the item which the skeptic is touching.
So the skeptic might touch a lamp and call out, “There is a chair, a sofa, a lamp, and a coffee table, and a rug.”
That is all the information the mystic needs to get the correct answer. The key is the word “and” in the list. Whichever item is stated before the first use of the word “and” is the item the skeptic is touching. Since lamp is the item before the first use of the word “and” the mystic knows the correct answer is the lamp. If the skeptic does not use the word “and” at all then it is the first item the skeptic calls out.
There will be party guests that suspect a ruse that are trying to figure out how you are pulling it off. With that in mind it is a good idea to agree beforehand that the first three times the skeptic calls out the list of items the rules should be ignored and the mystic will just guest a random item from the list. You want to get a few wrong at the beginning and not to use your pattern. This will make it harder for people to figure out what you are doing. After three times start to follow the rules and reel off five or six consecutive correct answers. This should be plenty to amaze your guests. As soon as the skeptic thinks the guests are suitably baffled, immediately stop the test. If you keep going, soon or later someone will catch on. So once the guests are astounded there is no reason to continue.
Now just sit back and enjoy the evening. It is up to you if ever reveal your trick.