Each $20 bill produced after 2003 contains a plastic strip embedded along its left hand side. When held up to the light you can see “USA.
Learn about counterfeit bills, plastic security strips, watermarks and The newest $20 bills printed by the Treasury contain three special security features of Jackson's portrait (as seen on the face of the bill) is "hidden" within the paper itself.
I can't remember if it will have a $20 watermark or not, but check to see if it has a thin strip with the word "Twenty dollars". Hold it up to a light.