Here are a couple of extra points of information on unmarked police cruisers. This information is primarily relevant to New Zealand, but should also be true in most places around the world.
Most unmarked cop cars have at least one extra radio antenna, usually mounted on the back. The exact length and description of the antenna depends on your local police force, so you are advised to check what antennas are on regular cop cars. Commonly, its a whip-style antenna slighly longer than a standard FM Radio antenna. Recent advances in antenna and radio technology have allowed the police to put the radio antenna integrated into the cars rear wind-screen. This makes them effectively invisible (By the time you can make the antenna out, you are far too close).
Obviously a bar of rollers on the top of a vehicle is going to give it away, so unmarked police cars need something else. Usually this is a set of flashers mounted inside the radiator grill, on the front dash and on the rear dash of the car.
If you spot the rear dash ones, they will either be the cheap red/blue combo, easily seen, or the mirrored style, which is harder to recognise. With the mirrored style, the lights simply look like small round mirrors on the rear dashboard of the vehicle, but when they light, the blue and red lights are rather obvious.
Your local police force may purchase a large number of vehicles at the same time. Though it is rare, sometimes they don't bother getting random plates, and all unmarked police cars in a locale have the same license place prefix. This is not as common as most of us would like, but Your mileage may vary.
These vary from location to location. The best advice I can give is go down to your local cop shop and start looking around. Look in the car parks outside the building if you can and try and spot an unmarked police vehicle. Once spotted, look for any other variations or markings that can facilitate spotting these vehicles on the road.
Mobile Speed Cameras
Speed Cameras inside cars are popular in NZ, previously the cameras were mounted in the back of white station wagons, and the boot (rear door) needed to be up in order for the camera to see you. This was a dead give-away when you were driving down the road and saw a white station wagon with its boot up. (On a side note: I had a friend who owned a white station wage similar make to the speed camera one and always left his boot up when he parked in on the side of the road).
Today however, the police use vans to mount the cameras. Why? For one, the cameras can now be used through the glass of the rear door, but the main reason is they can put a desk in the back and the police officer can catch up on his/her paper work . When a flash unit is needed for the camera (low light conditions), the flash unit is mounted outside the van on the tow-bar.
Please remember, this information is for entertainment purposes only.
If you are a safe law abiding driver, I recommend you use this information to spot the unmarked police cars so you can stay out of their way.
If you are a reckless unlawful hoon, please use this information to spot the unmarked police cars so you can drive in front of them recklessly, flip the birdy, and get yourself some more tickets for your collection.