MINNEAPOLIS - Counterfeit cash cases are on the uptick in Minnesota, according to Secret Service assistant special agent in charge Mark Johnson.

Every week the Secret Service’s Minneapolis Field Office receives more fake money, turned in by local police departments and banks that have confiscated the counterfeit cash from across the state, said Johnson. The numbers are up this year but are not at their highest ever, said Johnson, adding that the problem of counterfeit cash ebbs and flows year to year.

This month, one of those counterfeit bills showed up at Samaritan Way Thrift Store in Elk River.

"It was a $20 loss," said Sabrina Brown, a manager at the store.

She said she noticed the $20 bill was fake when she took in a real 20 and saw them side by side.

"It was more defined," Brown said of the fake bill. "It looked like a stamp-on almost, so I took it out and I marked it with the markers. It worked fine."

Brown’s currency marking pen failed to identify the bill as counterfeit. Secret Service officials say some criminals will bleach a $1 bill and print a larger dollar amount on the same piece of paper, creating counterfeit cash that passes the currency marking pen test.

"They're doing a really good job at the counterfeiting," said Brown.

Ultimately, Brown was able to determine the bill was fake by holding it up to the light and looking for the watermarked face and vertical security thread that appear in a real bill when light shines through it.

Just down the street in Elk River, Coborn's was hit, too. A spokesman for the store says fake $20s totaling more than $100 were used to buy a gift card and another item.

In Golden Valley, police say they've taken more than 20 reports on counterfeit cash so far this year, an increase from previous years.

"This year we have had quite a few more cases of counterfeit bills," said Sgt. Dave Kuhnly with Golden Valley Police.

Kuhnly says his department has made two arrests in connection with the counterfeit cases.

"What we're seeing a lot of is $20 bills," Kuhnly said.

In St. Paul, police say they've dealt with more than 50 counterfeit cash cases so far this year.

St. Cloud police say they’ve dealt with more than 60 cases of their own so far in 2017.

To know whether your cash is real, Kuhnly recommends buying a currency marking pen at an office supply store. For counterfeit cash that beats the marker test, Johnson recommends holding the bill next to another bill and looking for any differences between the two.

The Secret Service also has some tips for identifying counterfeit cash available online.

If you believe you’ve come upon a counterfeit bill, call your local police department, said Johnson.