Just as credit cards and online payment processing seem to be making cold, hard cash obsolete, the problem of counterfeiting is anything but.

In fact, the Secret Service – which is charged with investigating and preventing counterfeiting – says in the last fiscal year it helped pull some $182 million in fake US currency from circulation. That's more than double the previous year's.

The funny money problem is particularly severe for small businesses, who are more likely to be cash-based, less likely to have employees trained to spot bogus bills – and possibly already struggling financially.

The most commonly-faked bill? The $20. How to check if it's genuine? Just because it feels like a real bill doesn't mean it is one, experts cautioned the Los Angeles Times. Higher denomination fakes are often printed on real, lower-value bills that have been bleached first, says Jim Smith, senior vice president of sales and marketing at UVeritech Inc., a California maker of fraud prevention products.

Countertop UV machines – which reveal the fluorescent polyester thread woven into each legitimate bill - can cost $100 or less. (Each denomination glows a different color – a $20 glows green. A $5 glows blue, a $10, orange; $50, yellow; and $100, red.) Hand-held UV pens are even cheaper.

Whether you invest in high-tech equipment or not, check out the posters and other resources for spotting fakes here. The Secret Service website also lists some differences between genuine and fake bills.  

"You should know the simple things about [a counterfeit bill] that you can see when it's right there in front of you," Max Milien, a spokesman for the Secret Service, told AOL Business. "If at that time you can't make a determination, it will normally be discovered when it is deposited at the bank. The bank will catch the counterfeit note, then notify the business of whatever loss the business will suffer."

What to do if you think you spot a fake? The U.S. Treasury advises you not risk danger by trying to return the money if the situation looks iffy. Instead, try to get a good description of the customer and his or her license plate and contact your local office of the Secret Service. (Unfortunately, notes the Treasury web site, "There is no financial remunerationfor the return of the counterfeit bill, but you will have pride in doing the 'right thing' to help combat counterfeiting.")

Next February, the government will debut more protection for $100 bills in the form of color-shifting ink and plastic strips with 3D images.