The Warrenton Police Department is warning people to be extra careful when accepting cash transactions, after multiple businesses reported receiving fake bills over the last week.

“Since Wednesday, Jan. 2, we have taken numerous reports from local business of counterfeit ten and twenty dollar bills,” the police department said on its Facebook page. “Many of these were caught by the cashiers, but some were not.”

Anyone who takes cash payments for items they sell through online marketplace or buy-sell-trade pages should also be on the lookout, police added. 

Any fake bills should be reported immediately to local law enforcement.

There are several methods for detecting a counterfeit bill. According to an online article shared by Warrenton police, several methods can be done with just your eye and a light.

• Color shifting ink — $10 and $20 bills have a shiny 10 or 20 printed at the bottom right of the face side of the bill. This number is a reflective copper metallic color when viewed directly, but turns a dull green when tilted away from the viewer at an angle. Higher currency denominations have other color shifting elements as well.

• Printing quality — Authentic U.S. currency is printed with extremely fine, sharp detail. Poorly made counterfeits are not able to replicate this level of detail and will appear blurry, especially when looking at very small text on the bill.

• Watermark — U.S. currency is printed with a watermark at the right edge of the face side of the bill. On $10 and $20 bills printed in recent years, the watermark should be an image of the same face that is on the bill.

The watermark should only be visible when held up to a light. If it is plainly visible at all times, the bill is suspect.

• Security strip — Also visible like a watermark is a single vertical strip of small text somewhere on the bill. The text should say “USA” and the bill’s denomination, for example “USA TEN.” Again, this text should only be visible when held up to a light.

When exposed to a black light or UV light, this security strip actually glows a particular color. The strip on a $10 bill glows orange, while the strip on a $20 bill glows green.

Warrenton police have received multiple reports of counterfeit $10 and $20 bills.