It has to depend on the circumstances. Ordinarily (with some exceptions) unless you are actually under arrest you can walk away from somebody that you know is a cop, let alone somebody that you think might not be a cop. On the other hand, if you have just robbed a 7-11 and a guy in street clothes says “Police! Freeze!” it would probably be a mistake to say “you don’t look like a cop to me.”
This is an excerpt from a news story reporting the arrest of two different men who impersonated police:
“In a statement yesterday, Howard police said that residents may request to speak with a uniformed officer if they are stopped by a plainclothes officer driving an unmarked police car. Drivers can also turn on their hazard lights – to acknowledge the officer – and drive to a well-lighted location before stopping.”
This is from a D.C. police newsletter addressing the same issue (on page two, “MPDC issues warning…”):
http://mpdc.dc.gov/news/pubs/pdf/wn_030314.pdf
This is from a Colorado police statement on the same subject:
http://www.ci.erie.co.us/PS/imcops030303.pdf
The point is that a real cop will take reasonable steps to convince you that he is legitimate. But you can’t ignore him. If he thinks he has grounds to question you or arrest you, you don’t get some kind of immunity just by claiming that you thought he was a fake.
In general, offering to drive on to a well-lighted public place expresses your willingness to cooperate and at the same time protects your safety. If the guy has a cell phone you might ask him to let you use it to call 911 yourself. If you are behaving reasonably with a real cop he’ll probably be reasonable too.
But there is no one guaranteed right answer. If you really think the person standing in front of you with a badge, police picture i.d. and gun is an imposter, then you are looking at a pretty sophisticated level of criminality. Would someone like that really be lurking on a backwoods road waiting for a random passerby to stop and get out of his car? Is that really more likely than the possibility that he is a legitimate off-duty or plain clothes cop checking out a stranger? On the other hand, if you were in a shopping mall and someone claiming to be a cop tried to take you away from the crowds so you could “talk privately,” that might be the time to make a serious scene. You might just have to trust your instincts.
This is a link to a page of short Nolo Press articles on how to deal with police stops and related matters. Your specific question isn’t addressed, but there is a lot of practical information here.
http://www.nolo.com/lawcenter/ency/category.cfm/catID/3900BEB2-2599-4E9F-B5F09F0DF3E33C7B#D32FF664-6E30-4102-9B4F35007F022B40
This is a link to the ACLU’s “bust card,” on what to do during a police stop:
http://archive.aclu.org/pdf/bustcard.pdf