I don't think there's a supposed - it kind of depends on your appetite for risk, and how bolshy you are I suppose. I had somebody pull the line on me during a bit of an argument in public - I happen to know a fair bit about Policing so I asked him if he was, in that case, placing himself on duty at this point and therefore formally acting in his capacity as a Police Officer. I then asked him if he felt he had reasonable grounds to either detain or arrest or me.

Much as I'm actually very pro police, watching his smug little face drop when he realised his bullying tactics were going no place was very enjoyable.

Incidentally, most cops I know will do everything in their power to EVER admit to being a Police officer in public - it's just a sure fire way to get bad attention, and given plenty of criminals will happily try and kick the shit out of them in uniform, it's really not some magical card of self defence. Literally, the only time to say it is when you're putting yourself on duty at some incident.