“How do you know whether they’re cops or robbers?”
Yeah, like I have already explained, if you are a criminal and have recently committed a felony, it’s pretty much common sense that the men approaching your car with guns drawn telling you to put your hands up are probably cops.
The cops may suspect that they're approaching a criminal. They don't know that they're approaching a criminal.
When they're getting out of plain cars, wearing plain clothes, and pointing guns … to the honest citizen THEY LOOK LIKE CRIMINALS THEMSELVES.
The honest citizen will defend himself against them by any means necessary.
The cops had best learn to make damn well sure that they don't EVER look like criminals.