PORTLAND, Ore. – Karen Snell did the right thing when a man came into her empty bar one night and asked her to change a $100 bill for a stack of $5 bills.

She took the bill and put it in a special counterfeit bill detector next to the register. The small device uses a special ultra-violet light to highlight an identification strand running through the bill, and it showed up – if a little out of place.

Snell thought "close enough" and made change for the man, who then dropped $5 into a video poker machine, quickly played a few hands only to lose, and then left the bar.

Snell and her bartender then became suspicious. They checked the $100 bill again.

But then a quick test – inserting the $100 bill into the video poker machine – confirmed their fears. The poker machine rejected it.

It was a fake.

And while the suspect was caught on video, no arrests have been made. Snell was out almost $100 amidst tough times.

At the Portland office for the U.S. Secret Service, Resident Agent in Charge Ron Wompole showed KATU News a thick stack of counterfeit case files with bills ranging from near-perfect knockoffs to some very obvious fakes.

Wompole said the bills Snell received was most likely a smaller denomination bill, such as a $5 bill, that was bleached to clean it of ink and then reprinted as a $100.

Ordinary computer printers, especially those with high-quality abilities, can be tweaked to produce realistic-looking fake paper money. Counterfeiters regularly take advantage of that technology.

And since the paper used for the fake $100 bill Snell got came from an original paper bill, it still had the hard-to-duplicate security thread in it. It just wasn’t in quite the right spot for a $100 bill.

Wompole said another quick way to spot a fake is to check for a tell-tale "watermark," which is a hidden image, word or number that can only be seen when the bill is held up to the light.

All current U.S. currency has special watermarking to help foil counterfeiters.

Learn more about how paper money security measures and how to spot a fake bill by going to this story with an interactive guide from our sister station KOMO.

In Snell’s case, ifthe fake bill was originally a $5 bill, a portrait of Abraham Lincoln – not Benjamin Franklin, who is on the $100 bill – would have appeared in the watermark, giving the fake away.

Counterfeit investigators caution business owners not to put too much trust in the "special pens" used to spot fakes. High-end counterfeiters can work around that as well.

They suggest getting a UV device like Snell uses and taking time to educate themselves and their staffs on monetary security features like watermarks, security thread locations and the signs a bill could be a fake.

Wompole said the best way to spot a fake is to closely compare it to a bill you know is real.

Police recently arrested two men – twins – and their sister for passing fake $100 bills in the Portland area, but it is not known if they created the bill Snell received.