Just got a $20 back and you’re not sure about it? Pull another $20 out of your wallet and look at them both side by side. Since everything but $1 and $2 bills have been redesigned at least once in the last 20 years, this works best with a bill of the same series.
Check out the printing quality.
Real cash is printed with techniques that most counterfeiters are unable to duplicate. A good area to look at is the portrait. Are the lines sharp and distinct? Or do the fine details blur together and appear unsharp?
Pay attention to the serial numbers.
Do the numbers match? Are the numbers perfectly spaced and aligned, or are they a little wiggly? Those are two sure signs of a fake.
Also, if you got more than one of a denomination that you’re suspicious about, check to see: are the numbers the same on both bills? Every single bill has a unique serial number.
Look for the security features.
On all bills larger than $2, hold the bill up to the light and look for the security thread (a plastic strip running from top to bottom, embedded in the paper).
It’s in a different location on each denomination, so be sure to compare to another bill to see if it’s been bleached and reprinted.
6. Write your initials and date in the white border area of the suspected counterfeit note.
7. DO NOT handle the counterfeit note. Place it inside a protective cover, a plastic bag, or envelope to protect it until you place it in the hands of an IDENTIFIED Secret Service Agent.
8. Surrender the note or coin ONLY to a properly identified police officer or a Secret Service Special Agent, or mail it to your nearest U.S. Secret Service field office.
The Treasury also notes that there’s no reward — or reimbursement — when you turn in a fake $100 bill (or any non-legit currency), and adds, “but it is doing the ‘right thing’ to help combat counterfeiting.”