Adat by Maurice Janssen
Here is a quick and easy effect that is very visual. The performer requests a volunteer to think of any random digit between one and ten. Next, the performer directs their attention to a small piece of paper lying on the table. The participant is asked to name their number out loud and read the prediction. The piece of paper is totally blank. You introduce a small black light, ask them to announce their number and shine the black light on the paper, revealing your prediction was correct. [Note that in 2/3 of the cases you need to introduce a second piece of paper to complete your prediction.]

no switching
no discreet writing
zero maths, no counting
free choice of digits
one time quick, easy setup