Let me address Bob's post about being stopped by unmarked cruisers as well as search warrants...
First, most agencies whose policies I'm familiar with have restrictions on pursuits and/or stops in unmarked cars. This is only partly in response to impostors; there are also major safety issues given the reduced visibility of an unmarked car or even a slick top. More restrictions are generally added to plainclothes or non-uniformed stops. But those don't help the general public...
Generally, if you're concerned about the identity of a police officer stopping you, ask for ID. Assume that lights are valid, to the extent of stopping. You can roll the window down enough to talk, but not so far as to be in immediate danger until you see an ID or uniform. Ask 'em to call for another unit; few impostors have a backup impostor available! Most plainclothes officers will try to have a uniformed car make the traffic stop when practical. Call 911; realize that the 911 center may not be the dispatch center for the officer. (It's not where I work -- but the can contact the right cops pretty easily, or send one of theirs if there's any question.)
Search warrants are a different beast entirely.
Different states have different rules, but most view a no-knock or exigent entry as an exception to the rule. Some require that you establish separate PC in the application and affidavit to support the no-knock entry, while others allow you to make the call on the scene. Generally, there's going to be no mistaking an actual entry. It'll be loud, fast, but controlled. Typically, there will be a surprisingly large number of people, and there will often be a uniformed presence, though they may be outside during the initial entry. Yeah, I'm being vague. Sorry, tactical issues that could endanger a cop, and I'm not going to discuss 'em.
It's a tough call, and exigent or no-knock entry is usually done because it's believed to be the safest way to execute that particular search warrant. I firmly believe that all no-knock entries should be reviewed. If it's planned in advance, the reason for the exigent entry should be discussed, and agreed to. If it's a response to exigency at the time of the entry -- it should be reviewed and assessed later. The goal of the review should be to determine if it was necessary, and what can be done to control the situation better in the future.
I've done exigent entries. I've done 'em both as a planned entry and, more often, as a response to developments at the door. And I've done forced entries when the door wasn't opened for us. They're all rather scary to do -- because you've got a lot happening pretty fast. Our first choice is almost always to do a controlled entry after calling occupants out.