How does the marker that's used to check money to see if it's counterfeit or not?

The marker pen used to check currency will leave a black mark on the note if it is counterfeit. The marker pen works by checking the pH level of the bill or by testing for certain chemicals that should be present or absent from paper currency. For example, counterfeit cash may be produced on paper that has starch in it and rubbing the pen on the fake bill will leave a black mark. US currency is made from a cotton based fiber and contains no starch.