Federal counterfeit experts can usually tell the difference, and they've had plenty of chances to make the distinction lately.
"It's opportunity crime," says Jim Kollar, a Secret Service veteran. "But it's not tied into the economy, per se."
Gas stations are prime spots for counterfeit activity, said Kollar.
"It's a relatively easy place because you are not going to be looked at as much because there is so much activity at a gas station," Kollar said. "But we've arrested lots of people at gas stations through the years."
The group works with local law enforcement and businesses to assist them in detecting the fakes, which have a lack of water marks and security strips.
"It's a much flimsier feel, as opposed to stiff," Kollar said. "You can see the sheen on the 20 that shifts and you get different colors, where this counterfeit is going to be, anyway you shift it, one color."
Most automatic cash machines will be able to tell the difference, Kollar said, but it's a different story when a human being handles the bills.
"They have given us many, many fake bills in the past," said Darius Ghaffari, an Arco gas station manager.
Ghaffari said he has added special protective measures.
"They are cash accepters," he said. "They have electronic eyes that can tell the good guys from the bad guys."
While technology has made it easy enough for a person with a 3-in-1 printer to copy a bill to make fake cash, Kollar said the government stays one step ahead.
"We go through redesign of the bills periodically because people start figuring out that there are security features and micro printing."