Problems with fraudulent banknotes, This Money checker detector pen is suitable for all currencies. Mark a bank note With a genuine note - the pen ink will turn a clear to yellow colour, which will then either vanish completely or leave a clear mark after a short time depending on the temperature, A potential forged note - the pen ink will turn a brown to grey colour which will remain on the notes,  Potentially forged notes should be checked with an additional device such as a bank note testing machine at your local bank.