Just a little bit of thought should answer this for you.

Hint: How would a sting or other undercover operation work? Do the police lie during interrogations?

It's folklore, based on a misunderstanding of the concept of entrapment. Learning exactly what entrapment means may be worthwhile, but cops lying about being cops does not constitute entrapment.

asking if someones a cop, or stating, 'cops can't come' shows intent.

> does anyone know if you can just ask them if they are a cop; with them having to respond truthfully?

Of course police can lie about being police officers. Another person kind-of answered already, but this apparently needs to be stated unambiguously: A police officer can lie to you about being a police officer and get away with it.