You can usualy tell the undercover ones because they usualy have more arials.
not any more, they use same systems volvo's started, the copper wire in the windows, allow you to have many areals on one plain of glass so you dont attract attention etc
also alot of the cars arent standard so will have more bhp than book models, which are obviously undesclosed because they dont want you knowing there secrets
I know that the CID cars, and the panda cars (Mostly 1.6 Diesel Focus' - honestly, who are they going to catch with these lol) have a tiny 2nd aerial on the n/s/r of the roof and a small disc, (looks like a hockey puck)
Also with regards to teh cars having more bhp than standard is enerally considered a myth, i know a couple of peoiple that work at HQ in Durham who service, repair and kit out most of the cars and they've all said that the cars are bog-standard with no engine modifications. Even on the T5 owners forum i used to use when i had my 850 T5, alot of people on there rock ex-police cars, and not one of them has ever produced a higher power output when on a dno compared to a standard model