ST. AUGUSTINE, Fla. -

The St. Johns County Sheriff's Office is investigating four cases of counterfeit currency passed at area businesses over the five days.

Counterfeit 10s and 20s have been passed to three businesses and at a garage sale in the southern part of the county last weekend. Witnesses said the money was passed by men and women.

"It seems pretty sporadic," said St. Johns Sgt. Catherine Payne. "There's a lot of things it could go on with this, especially it being the Florida tax-free weekend."

The confiscated bogus bills are being held as evidence as detective search for whomever is responsible.

"Then we have to get down to the bottom of this ... Was this intentional and the suspect knew they were passing on something counterfeit," Payne said. "Or (did they get) the counterfeit money in some type of transaction throughout the county this weekend?"

Counterfeit money investigations are handled by the Secret Service. A spokesman said they are aware of what's happened in St. Johns County.  As a prevention they suggest people look closely at their cash.

The Secret Service webpage has tips on how to spot counterfeit money. The best advice is to closely look at the portrait on the bill. On a real bill the hairline is clear, but on the counterfeit, it's less defined.

Also check the treasury seal, which should be clear distinct and sharp.  This will appear a little blurry on a fake bill.

The borders also show some problems.  Rather than it being clear and unbroken, these may be blurred and indistinct on a fake bill.

If you do have a counterfeit bill, there's really no way to get your money back. Police suggest don't try pass it off because you could get in trouble.

One of the bills in each set pictured below is counterfeit.  Can can you tell which one?

Answer: The top bill is in each pairing is real; the bottom is a fake.