Printing. US bills have raised printing, and all lines needs to be sharp. Hunt for irregularities in definition between bills in addition to lines wavering in thickness, coloration or clarity. Federal Reserve Notice paper is made from ¼ linen and ¾ cotton, and feels a bit tough to the touch https://thekiwisocial.com/story2347260/the-fact-about-undetectable-counterfeit-bills-for-sale-that-no-one-is-suggesting