This is a fairly simple trick which can be used to determine the particular type of an item within its class, and some particular number associated with the item. Thus, a card: determine what suit and what number it is. Or: a coin: whether it's a penny, or a nickel, etc., and what date does it have; or paper money - its denomination and serial number.
The performer introduces his assistant as someone who can receive his/her thoughts on specific subjects — as mentioned above. S/he blindfolds the assistant; but stating that it's common knowledge that blindfolding can leave a chink of vision, the performer sits the assistant down with his/her back to and beneath a big screen controlled by a PC, which the audience can read but which is so positioned as to make it obvious that the assistant can't. (The original description of the trick requires a blackboard: but these are modern times. Still, for an impoverished company, a blackboard will work just as well).
The performer asks an audience member to lend him/her a common item which has some number on it. S/he thanks the person and says something about the item ("this is a coin", etc.); at the same time, the performer (or another assistant) types the denomination ("dime") and the number (the date) on the PC: this is so the audience will know what's being sought). S/he then asks the assistant to identify its denomination, and then the relevant number. The assistant answers all the questions (with various hesitations - this, after all, is a mental exercise requiring great concentration!), and as s/he does, the performer types the answer on the PC keyboard so that the audience can see the results.
There are two components of this performance.
First, the performer and the assistant must have practised counting at a standard rate, so that after a specific interval of time they will arrive at the same number. This must be slow enough to be definite but fast enough so that there isn't a huge time difference between counting to three or to nine.
Second, there must be a series of key sentences, or key phrases within sentences, or even a particular way of saying the sentences, that will give the assistant the information s/he needs to identify the category of the item. With a deck of cards, four such keys are needed: such as:
"here's a playing card" means clubs
"this is a card" means diamonds
"here we have a card" means hearts
"OK, let's play cards" means spades.
(If the trick is performed twice in a row, "a" can be replaced by "another" or by any such innocent substitution).
The assistant concentrates and after a pause gives the suit: the performer types it in on the PC — not immediately but after the mental count for the first number. Crucial! This count starts after the assistant has spoken. The performer starts typing once the correct mental count-time has elapsed. The assistant then knows what the number is, and can speak it a second or so after the performer has stopped typing. Since this example is for cards, either the assistant is right or not and the trick is ended in either case.
For something more complicated, such as a coin, more informative phrases are needed but the number recitation is the same. The assistant gives the first answer, the performer starts typing after the appropriate time interval, the assistant gives the second answer, and so forth. The wait between the time the assistant stops speaking and the performer starts typing is the key to the whole thing and consequently the two persons must be absolutely in time!
For an item with many digits to identify, there's an additional wrinkle: it might become obvious that the length of the pauses has something to do with the number pronounced; so each alternate number should be counted BACKWARDS.
A dozen or so introductory phrases can be concocted which read the same except for the item identified. Thus "this is a coin"; "this is paper money"; "this is a cell-phone" etc. (although the last might be tricky! There are so many!) In such a case the list of items is mentally switched to match the key phrase. If the item offered is something scarce, or if the list of phrases can't accommodate it, some defensive move will have to be fallen back on. Houdini, when presented with a handcuff that he wasn't familiar with, used to reject it with the chilling phrase "non-standard", and his personality was such that nobody dared press him. It might be a good idea to pre-screen the items presented.
The real purpose of the PC or the blackboard is that the blindfolded assistant hears the chalk or the typing and knows therefore when the count has stopped. The PC must have a loud enough keyboard click to be heard above normal crowd noise, or perhaps a radio link that vibrates when a key is touched can be placed in the assistant's pocket; although the performer can call for silence. (If a blackboard is used, the scratch of the chalk ought to be loud enough!)