Pack of 10 handy banknote Tester Pen for simple, portable protection against counterfeit notes; Tests Euro (also all new certificates, US dollars, British Pounds How it works Mark the note with the pen, preferably at the edge note Wait a moment Genuine: The Line fades to a light yellow colour and is difficult to make out later. Fake: The Line is very dark and easy to make out Banknotes are made from a special type of paper then pen thanks to the composition ink, recognises the fade. Counterfeit notes are often made from normal paper, which causes the ink to react and turn dark