How might this be done. In simple terms you could, for example, show the subject a coloured shape, perform the procedure and ask them to recall the colour and shape. If they can't, bingo. But this assumes the subject is not lying for the benefit of the testers. So, you give them an incentive to recall the colour and shape; a big payout or the promise their kneecaps will stay in the same place. But the incentive option doesn't apply here, for reasons I won't bore you with, and the procedure relies on the subject's trust.