Take a supposed policeman to a very well-lit and crowded area if he approaches you--especially at night. Fake policemen don't like to move to areas where they can be easily identified and restrained if they're up to no good.
Demand that people who seem to be authority figures provide identification. Fake policemen will likely not want to provide you fake identification that you can inspect closely.
Watch out for fake policemen that demand to see identification, passports and other information contained in a wallet. Fake policeman will grab your wallet and the documents and run once you pull them out and show it to them. A common way that fake policeman encourage you to show documents and a wallet is a charge of using counterfeit money.
Know that fake policeman may hang out at airports to try to make you do things that you wouldn't do normally. Fake policeman have been reported to usher travelers to scam hotels after fake bomb scares, steal personal documents and extort money from travelers.
Note apparent accomplices. Fake policeman often have accomplices in plain clothes. Some of these accomplices act like tourists and strike up conversations with you. A typical scenario is that a fake policeman will request a passport and a wallet from the other tourist to check for counterfeit money. When you hand over your passport and wallet, they run.
Realize that fake policeman can also be on the road. They may try to stop your vehicle and search your car for items that they can steal.
