Good news: You’ve finally found a buyer for the old clunker that’s taking up space in your driveway. Best of all, he’s going to pay you cash. Before he does, though, read on. Although passing counterfeit money isn’t a huge crime in this country (thanks to the Secret Service, which was established in 1865 to deal with what at the time was a huge problem), it’s always possible that someone will inadvertently — or even purposefully — hand you some fake funds.

Related: Protecting Yourself From Predatory Debt Collectors

Not long ago, spotting a counterfeit bill was nearly impossible. Recently, though, the government has taken security measures keep fake money out of circulation.

With the exception of the $1 bill, the Bureau of Engraving and Printing has revamped most paper money at least once.

What’s in your wallet? 5 clues a bill is counterfeit

The Federal Reserve suggests using a special pen that can detect if paper money is counterfeit. If a bill is fake, the pen leaves a black mark. You can find such pens at office supply stores and online.

Whether you have a fake-money pen or not, it can pay — literally — to know how to detect counterfeit cash. Take note of these signs that a bill isn’t real.

Get a feel for it. Most U.S. money is printed on paper that’s made of cotton and linen, giving it a texture that’s somewhat different from regular paper. Genuine currency paper also is embedded throughout with tiny red and blue fibers. Many counterfeiters print red and blue lines on their paper but it’s easy to feel that they aren’t embedded.

The $100 bill, which is the most counterfeited abroad, was revamped for the third time in 2013. One of its several new security features is raised printing. Glide your fingers across Benjamin Franklin’s shoulder. You should be able to feel the raised intaglio printing.

Watch the shades shift. Several denominations of paper money have color shifting ink, including $10, $20, $50 and $100 bills. When you tilt such a bill back and forth you can see the number on the bottom right of the bill switch from copper to green.

Find the watermark. One of the best ways to determine whether a note is authentic is to find the watermark, according to the Fed. Hold up a bill to the light. On both sides of the note you should be able to see a watermark of the person whose image is on that bill. For example, on a $10 bill there’s very faint image of Alexander Hamilton (who wasn’t a U.S. President, by the way) on the far right side of the front of the note and on the far left on the back.

Follow the thread. All bills worth $5 and more feature a security thread that runs vertically to the right of the portrait on the front. It also can be seen on the back. The thread is imprinted with a pattern alternating how much the bill is worth (on a $10 bill this is spelled out as “ten”) and an image. If you can’t see it, the bill is most likely a counterfeit. The security thread on the new $100 bill is in 3-D.

Study the serial number. Serial numbers have a distinctive pattern and style. The digits are evenly spaced and printed in the same ink color as the Treasury Seal right above it. If a bill is counterfeit, the numbers will be a different shade than the seal and/or won’t be uniformly aligned. Keep in mind too that serial numbers are a unique combination of numbers. If you see the same number on two or bills, they’re money is fake.

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What to do if you encounter a counterfeit bill

The Department of the Treasury advises taking these steps if you find yourself in possession of a fake note:

  • 1.Do not put yourself in danger or give the bill back to the person who handed it to you.
  • 2.If possible, delay the passer so you have time to note any distinctive physical features to share with tell the police.
  • 3.Contact the police or your local Secret Service office.
  • 4.Write your initials and date on the white border of the bill.
  • 5.Handle the bill as little as necessary. Place it inside a plastic bag or envelope.
  • 6.Give the bag or envelope containing the fake bill to an identified police officer or Secret Service agent.

Related: 7 Costly Money Mistakes and How to Avoid Them

Muriel Vega is a writer with a passion for budget travel and staying safe while abroad. A Georgia State University graduate, she has over 6 years of editorial experience and has written for The Guardian, The Atlantic, The Billfold, among other outlets. In her free time, you can find her baking pies, playing with her two dogs and cat, or planning her next vacation. She spends way too much time on Twitter, one of her favorite social media channels. Her favorite safety tip: Make sure you have all the necessary shots before you go abroad.