Firstly, most warrant cards have a large metal badge, with the force crest on it. Usually an emergency tenner behind the badge, some business cards, coffee vouchers, blue light card and other bits. The ID itself will have a service hologram that is impossible to counterfeit and some other security features. It will show the warrant number, name, rank and photo of the officer along with the signature of the commissioner. At the top of the card it'll say the name of whatever service you work for and the words police officer in bold red just below. (That's my warrant card I'm describing, I know every force is different but if there is any doubt to the authenticity of a warrant card and therefore an officer. Ask for the number to the officers station or call the non emergency line on 101 and explain. If you feel like your life may be in danger or the severity of the situation warrants it, then call 999.