Though it seems too obvious to even try, apparently many people impersonate the NYPD by wearing NYPD hats or shirts. Newsday looks at the problem of selling NYPD-logo emblazoned clothing, as the NYPD recently shot an armed man in NYPD gear in the Bronx. The conundrum is that the NYPD is trying to beat cheap imitations of NYPD clothing, which has been popular post-September 11, but the clothing also becomes must-haves for "criminals who commit home invasions and robberies by passing themselves off as cops."
The New York City Police Foundation, which sells NYPD clothing and accessories (check out this duffle bag) online, says, "There's always a risk somebody could cobble things together, a shirt, a hat, and try to fool somebody. But we're not selling shields. The items we're selling are available anyway." However, the NYPD also thinks the NYPD logo has become as popular in some circles as sports teams' logos (quick, get Perello on the job) and one detective says, "Every day, we run into guys with a PD hat or a sweatshirt. People think they're cops and let their guard down and wind up getting robbed."
If you're ever in the unfortunate situation of a police officer demanding to enter your apartment, first, demand to look at the police shield. Also ask for a card. It's important to detect a certain kind of world weariness, a la Detective Lennie Briscoe. If they have an NYPD hat and sweatshirt on, plus they are holding an NYPD teddy bear, it's probably an impostor. If you're still suspcious, start yelling about your lawyer, calling the ACLU and the Post.