Usually when someone is saying he is an APD officer, i tell him that i will be searching for papers or badge, if non found then I will tell them that they can't be seen as an officer. If they say they were an officer and they say the guidebook should say different things, I would say the classic : Well Mr. Jeff, alot has changed since your betrayal.
Usually idc if they impersonate unless they threaten me with APD actions. I just try to RP it out, in the end it's just a game.