There is really now way possible to prove to someone they work. They can always fall back on....."someone is moving the pointer". But if you have two people playing, if only one is purposely moving it, it doesnt move straight if you are lightly touching it. The one pressing hard to move it, their end of the pointer goes first and crooked. (I tried it to see the difference)
When Brooke and I used it, it went off the board to one corner. Now I just read (not that I believe it or not) that when it does that, the spirit has escaped the board, and is in the house.