WESTMINSTER, Colo. -- Westminster police issued a warning about the dangers of counterfeit money Sunday after a Girl Scout group reported they unknowingly accepted a fake $100 bill for cookies.

The girls were selling cookies at the King Soopers at 100th Avenue and Wadsworth Parkway.

The couple, described as being in their 20s, used the fake $100 to get change, said police spokesman Trevor Materasso.

When the Girl Scouts tried to deposit the $100, the bank told them it was counterfeit.

“It’s frustrating anybody would do it to anybody in the first place. To do it to children is just one step lower,” said Jil Hennessey-Seabolt, cookie director for Troop 2121.

Since the incident, though, the community has been extremely supportive, she said.

Several people left donations for the troop at the King Soopers and an anonymous person called police and made a $100 donation.

Westminster police issued the following tips to spot counterfeit money:

Don't accept bills larger the $20.

Use an identification pen to mark the bill and confirm it is U.S. currency (If it has a mark like it has been checked, check it again, criminals will add a fake mark so you don’t check it).

Check the dimensions of the bill against another bill known to be verified (often counterfeit bills are either larger or smaller than actual U.S. currency).

Check to be sure the print is aligned properly on the bill, as counterfeit bills are often mis-aligned.

Look for the security thread (new $5 and larger bills have this).

Look at the detail of the bold bill number 5, 10, 20. The edges should be very sharp and precise (most printers won’t match the detail quality of U.S. currency).

If in doubt, ask them for a different bill or another method of payment.