Quoted from Bugsy:
Or just buy a counterfeit bill pen. I had a seller use one of these when I bought a game and thought it was a great idea.
amazon.com link »
A couple of weeks ago one of our local banks had a notice on the front page of their website, as well as the front page of the newspaper, warning of counterfeit money going around not able to be detected with the pens. I'd always used one, but I've started looking for other identifying characteristics.