>'MrMouse says his single-ply bills do not have magnetic ink, and so they won’t pass machines designed to look for the presence of this feature.'
Clever tip for detecting counterfeits, found in comments:
>'A good strong, neodynmium cylindrical magnet stuck to the side of something is a great way to check US currency. Just hold a bill by the corner and dangle it close to the magnet. The bill will show a slight, but very observable attraction. Easy and fast.'
This is a key point. Detection by human is fairly limited as they only use their own, easily fooled, eyes and a detection pen, which can be fooled if starch-free paper was used by the counterfeiter [1]
Clever tip for detecting counterfeits, found in comments:
>'A good strong, neodynmium cylindrical magnet stuck to the side of something is a great way to check US currency. Just hold a bill by the corner and dangle it close to the magnet. The bill will show a slight, but very observable attraction. Easy and fast.'