Printing. US bills have elevated printing, and all lines needs to be sharp. Try to look for irregularities in definition among bills and lines wavering in thickness, color or clarity. Experience the area on the bill to verify the paper is a bit tough as well as ink is lifted. https://totalbookmarking.com/story17074308/the-fact-about-undetectable-counterfeit-bills-for-sale-that-no-one-is-suggesting