Simon Calder: When is a fake banknote not a fake?

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The search for a perfect murder is the constant quest for crime writers; but for petty villains in South America, it appears that the quest for the perfect scam for relieving travellers of their excess cash has reached a perfect conclusion. It relies upon the large number of forged bank notes in circulation, but is far more subtle than simply handing over fake bills in change.

For the would-be scammer, the tough part is getting a job in a Peruvian bus station selling tickets for one of the many competing companies. Then it's just a question of timing. Bus terminals are notorious hotspots for crime, especially pick-pocketing and snatching bags. This scam, though, is almost impossible to detect.

Here's how it works. A gringo passenger turns up and buys a long-distance ticket. He or she pays with a 100-sol note, worth £20. The ticket vendor knows exactly which bus the traveller will be on, and what time it leaves.

Three minutes before departure, he leaves his desk and goes to the bus. He asks the passenger to step down from the bus for a moment because a problem has arisen. The issue, he explains apologetically, is that the 100-sol bill the traveller used to pay for the ticket is a forgery. The villain shows the flaws, and hardly needs to explain the issue: he needs a replacement, and quickly – because the bus is about to leave.

The traveller is understandably aghast at inadvertently passing a counterfeit note. The subsequent reactions go as follows:

1. "Which rogue supplied me with the fake?" as the traveller casts back to recent transactions at banks or bureaux de change.

2. "How awful to jeopardise the poor fellow's finances. I bet the company makes him pay for any forgeries."

3. "The bus is about to go, and my luggage is in the hold. I have to sort this out soon."

4. The traveller reaches for his wallet and pulls out another 100-sol note.

5. The villain's expression changes from a concerned frown to the epitome of blessed relief. He hands over the fake in return, shakes the traveller's hand and walks back to his ticket desk.

6. The bus has barely started on its staccato journey, leaving the terminal behind in a cloud of poorly combusted diesel fumes, when the next three reactions emerge.

7. "Hang on, that ticket seller handles large notes day in, day out, professionally. So surely he should be able to spot an obvious fake."

8. "I bet he has a secret stash of fake notes at his desk, and brings one out whenever he serves a gullible foreigner."

9. "Bother!" (or possibly a stronger reaction). "That 100 soles would have kept me on the road for an extra day."

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The beauty of the scam is that there is no downside. Some prospective victims may be either unwilling or unable to hand over good money in return for bad, in which case all that will happen is that they will spend the rest of their trip feeling bad that they created such a dent in the poor man's finances. In the unlikely event that the traveller returns later in the trip to the same bus station, and decides to make a fuss, it is simply his word against the villain's.

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So how to avoid becoming a victim? First – and this is a good policy when spending large bills anywhere in the world, whether you are paying a taxi driver or a bartender – say out loud the value of the note you are handing over, such as "Cien soles" (100 soles).

You could invite the recipient to study the note carefully, so that any irregularity can be spotted at once rather than after you have left the scene. But if, despite these precautions, you find yourself in the uncomfortable position of being falsely accused of passing fake notes, stand your ground. Call the villain's bluff by offering to go to find a policeman. Which is easy to say on a summer day in Britain, but trickier to do in the Andes.

Attack of the clones

A good way to avoid cash-based scams is to use a credit card. But then you run the risk that your card will be "skimmed", and used for illicit transactions that you will be unaware of until you return home or (more likely) your credit limit is crashed.

Hitherto, I imagined that this was a rare event. But Rob North of Billingshurst in West Sussex tells an extraordinary story. "I've never held a credit card, but on a holiday in South Africa the car-rental company insisted I supplied one as a guarantee."

Mr North duly asked his bank for a credit card. He used it once, at the car-rental office in Johannesburg. Since then, his statement describes an astonishing itinerary, starting in South Africa then popping up with transactions in the US and Europe.

"The villains have racked up hundreds of pounds on a card I never wanted," he says. This is a reminder that cash, for all its pitfalls, says less about you than American Express ever can.