Undercover police will not disclose their true role even if asked.

Finding out if someone is an officer of the law will vary in difficulty depending on that person's role in the police department. For example, if the person is working undercover, finding out if he is a cop will be much harder than if he is a beat cop or detective. Most officers are proud of the profession and happy to talk about it with those who ask.

Ask her. The most direct way to find out if a person is a police officer is to ask her. Ask politely and respectfully. Do not use the word "cop" as that can be taken as pejorative. For example, "What are you? A cop?" can be offensive. Instead ask, "Are you an officer of the law or working with a law enforcement agency?" Contrary to popular belief, a police officer does not need to disclose that she is a cop.

Call the police station. Every police officer in a city has a station from which he works. If the name of the person in question is known, call up each police station and ask to speak to that person. If the person is not known at any of the stations in the jurisdiction then he is probably not a police officer.

Hire a private investigator. A private investigator will follow the person in question around and collect evidence of her doings. If the person is a police officer, she will likely have contact with other officers or check into a police station.