Handy banknote tester pen for simple, portable protection against counterfeit notes; tests euros, US dollars, British pounds Mark the note with the pen, preferably at the edge 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. (See images.) Banknotes are made from a special type of paper on which the pen's ink will fade due to its composition. Counterfeit notes are often made from normal paper, which causes the ink to react and turn dark