After the detectives identify a subject called Mooney, a detective tells them she made an NCIC (National Crime Information Computer) check on him and he is "on the job" (working for the NYPD). According to the FBI, the NCIC system is only used for functions such as apprehending fugitives, locating missing persons, locating and returning stolen property. It would not contain employment records.
If the subject Mooney was indeed "on the job" as the detective stated, it would have come up in a normal background search. That someone is a police is only a "secret" if the officer is undercover; normal police officers in normal roles can easily be identified during a normal (not NCIC) background search.
While doing a background check on Mooney, the detectives find he is "on the job" (working for the NYPD) in Queens. In a subsequent scene when the detectives are interviewing him, Officer Mooney is shown wearing "27" insignia on his uniform. The 27th Precinct is the one that the detectives work out of in Manhattan.