How to spot counterfeit bills before you're a victim

When the Secret Service started in 1865, its sole role was to investigate counterfeit money. Back then, about one-third of all US currency was fake. 

"We estimate in circulation today that one in 10,000 notes is counterfeit," said Douglas Zloto, special agent in charge.  

Zloto says they just busted a scam in the Detroit area.   

"He was using an all-in-one printer out of the back of his car and he had people he'd sell counterfeit bills to," he said.  

You could fall, victim, if you're doing online person-to-person sales - or garage sales.  

Zloto says, first compare the money you know is real - with new money you receive. No two serial numbers, for the same denomination are the same.   

"If someone hands you two bills of the same denomination they have the same serial number, I can guarantee you one of them is counterfeit," he said. 

Bills $5 and up have a watermark and color-shifting ink based on the light.  

"It goes from green to yellow," Zloto said. 

The ink is printed on the paper and you can feel it - "By rubbing Franklin's shoulder you can feel the ink is raised." 

And the security thread takes a different hue under a black light that is nearly impossible to replicate.  

"Under the black light the security thread grows a pink color," he said.

Zloto says getting a fake is an unfortunate game of hot potato. 

"If you are handed counterfeit money it is not worth the paper it's printed on," he said. 

Zloto said that the most popular bill is the $20 that counterfeiters attempt to pass.