If a man in a bar offers you a laptop for $70, you know it's probably stolen.
Yes, he might be wearing glasses and look a little like Bill Gates, but, please think, it's probably stolen.
Similarly, if someone tries to sell you an iPhone for a radically reduced amount, suspicion should surely be your guide.
Police in San Francisco are now using a slightly suspicious method to test your suspicions to the full.
Officers in plain clothes (which presumably means plaid shirts and 7 For Mankind jeans) are wandering around areas known to be popular for stolen goods purchases. These officers are offering iPhones that they openly declare are stolen.
They're testing you, you see. They're testing you. If you say "yes, please," they'll be arresting you.
The idea, as the Huffington Post reports, is to kill iPhone theft by squashing the market for the phones.
As San Francisco Police Capt. Joe Garrity told the Huffington Post: "If they steal the phone but can't sell it, there's no market. We're cutting the head off the snake."
Police claim that the snake is of a strong and venomous variety. Fifty percent of all thefts in the city are smartphone thefts. Many of the victims.
It wouldn't be hard to imagine that not everyone admires this police tactic. It seems as if the undercover officers are luring people to commit a crime.
These people might wish to buy phones that have, say, accidentally fallen off a truck or flown through the air on a windy afternoon.
Police, though, claim that the area in which they operate -- the corner of 7th and Market Streets, very near Twitter's HQ -- is notorious for buyers who know exactly what they're buying and are linked to the global trade in stolen iPhones.
Indeed, San Francisco is said to be the hub of the stolen iPhone network.
In a touching element of casting, one of the police peddlers, Officer Tom Lee, used to work in an Apple store. He even tells potential customers that his phones were stolen from a nearby Apple store.
The legal trick is to never suggest a price. He asks customers to make him an offer that he cannot refuse and his waiting, armed co-workers cannot wait to hear.
Lee said he has had offers ranging between $25 and $200.
It's unclear how effective such sting operations are. San Francisco District Attorney George Gascon, himself the former police chief, isn't convinced.
He told the Huffington Post: "This is like the drug war -- the more arrests you make for drug use, the more drug use seems to go on."
How odd to hear the iPhone being compared to drugs.
Are iPhone users so dependent that they always need another fix? Does that explain in part why constant disgruntlement currently wafts around Apple, like the peculiar smells at 7th and Market?