No. If they did, there wouldn't be any narcs. Cops can legally lie to you. People that say yes either don't know or want you to get ARRESTED. Cops want to arrest everybody so anybody that answers yes might be a cop.
Im not 100% but in most cases yes. A police officer will announce he is a police officer and should have his/her badge in clear view. one exception i can think of is if the officer is doing a...