I hand them a slip of paper and a pen and ask them to wrtie it down. If they do with no questions, I peek it or whatever and reveal.
Now, some of you may be wondering what to do if they question writing it down. I very simply tell them that I'm having them write it down because they obviously want to try and bust my balls. So, even if I got it right they could lie and blow the whole thing.
If they are going to try and challenge me, they are going to follow my rules. If they don't wantt to, to hell with them. I just move on after that.