As far as devices that attempt to actually read the human mind go, the best I know of are called functional magnetic resonance scanners, but they're quite expensive. They range from $1 million to $2.3 million, and cost a couple of thousand dollars per performance.
And they don't give a particularly clear image of what the spectator is thinking. And the spectator might get rather suspicious because the gimmick is massive.
You're probably better off going down the "writing stuff down" option. The best gimmick I've personally come across is the Mascot Moth
, although there are others. Do a search on the forums for the word "impression".