http://images.mentalfloss.com/sites/default/files/istock_000084782439_small.jpg?resize=1100x740

The quickest way to check is to feel the paper. Real US bills have a slight texture from the printing process.

https://i.imgur.com/5hrztCk.png

US bills $10 and higher have color-shifting ink. The lower-right denomination should change from copper to green. When tilted, its pattern moves left and right, up and down.

https://i.imgur.com/ryGvf41.png

Hold the bill up to the light. For any current $5 bill and higher, look for:

1. The watermark of the same face as the portrait side of the bill.

https://i.imgur.com/TnghivC.png

2. The security thread, in a different place on each denomination. It will also glow under UV light and the thread of each denomination glows a different color.

https://i.imgur.com/TnghivC.png

New $100 bills also have a 3D security ribbon.

https://cbsnews3.cbsistatic.com/hub/i/r/2013/09/25/95cb0656-360b-11e3-8ce8-047d7b15b92e/resize/620x/f59b354497f6a9f3cec2e8986dbc313c/new_bills_AP453657463085.jpg

To keep counterfeiters on their toes, the US Treasury continues to develop new security features.

Who knows what tricks they’ll come up with next?

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