[ARCHIVE ANSWER]
One of the most dangerous wives tales to citizens says: An undercover kop must tell you s/he is a kop if asked. This is a lie. Even when asked, undercover kops will never tell a person they are law enforcement.
There is no law or policy stating a kop must reveal who they are! All the covert training classes teach officers to never reveal their identity. I don't know where that bad information got started. While working undercover, I have had several suspects ask me if I was undercover. The suspects would even announce I must tell them if I were a kop. Of course I never revealed my undercover status. I lied, and they got busted.