Okay, how do I know that person is a policeman or not? If you don't follow their orders, they'll punish you and if you follow, there is a chance that they are actually criminals.
Use common sense and know your rights. If they knock on your door telling you that they have info your money might be counterfeit and that you have to give them everything that you have asap, you can be pretty sure its bullshit as these things don't work that way.
In that case, government should pay for every damage that victim has faced by fake policeman because if the government has the problem where some people can get police uniform, guns, police car and move in the streets, definitely this country is not doing well.
You don't need real uniforms, police cars and guns to pretend that you are police. Fake badge is usually enough and that's why these scammers usually target older people who are more gullible, easy to confuse and won't resist or make fuss.