This is the pocket version of the well known "hidden nail under the cup" trick.
The mentalist shows three black envelopes and three blank cards. He asks a spectator to choose one of the cards and to draw a nail with a marker. The spectator then mixes the envelopes containing the card with the drawing of the nail and the other two blank cards and puts them in a row on the table. So the mentalist, without error, will find the envelope containing the "nail"!
The effect is made with care. The marker is not included.
It uses a very simple idea (no magnets, electronics, etc.).
It can be repeated for many times. Everything can be examined before and after the presentation.