FORT WORTH (CBSDFW.COM) – For many of us it seems like money just passes through our hands. But, have you ever stopped to actually study those Jacksons, Grants or Ben Franklins?

More businesses in Fort Worth are noticing fake $20, $50 and $100 bills. CBS 11 News talked with some of those storeowners to find out why the funny money is so hard to detect.

Sammy Alaqqad says any customer who walks in the door of his store could be about to commit a crime and not even know it. “Some people carry it [counterfeit money] in their pocket and they can’t tell,” he said.

While at the store Alaqqad showed us a $20 bill with black marks on it, it’s counterfeit and someone had passed in the store.

The black marks were from a special pen that shows a yellow mark when touched on real $20 bills. But Alaqqad says he’s had fake bills that the pen marked as being real.

“Now they got printers they make money that looks 100-percent real,” he said.

The Great Outdoors Sub Shop staff heard counterfeit money was circulating and posted signs warning customers that no $50 or $100 bills would be accepted.

“A couple of weeks ago we put our signs up and, as a matter of fact, that day we received a [fake] twenty dollar bill,” recalled store manager Marco Rosas. “So, we’re staying on our toes about it.”

The U.S. Secret Service says it hasn’t noticed any spike in the number of counterfeit cases they’re investigating. But agents say once fake money is passed in the area it can circulate for a while before it’s noticed.

When it comes to the fake cash and local storeowners, it’s literally money out of their pockets.

“We give them the food. They pay for it. We take the money to the bank,” explained Rosas. “The bank says its counterfeit. They [the bank] take that money and we’re out the money and the food we gave away.”

Alaqqad says its just plain thievery. “Especially when you get two or three hundred dollars. That’s lost. They take it. No credit.”

Police and storeowners advise you to check your cash before going to the store and make sure you’re not inadvertently passing off counterfeit money.