Here in the UK, government organisations are regulated in term of surveillance, there is no issue to overtly observing someone as the person being observed knows what is happening, hence why there are always signs advertising the use of CCTV or speed camera equipment.
The same goes for police cars, there are video cameras recording the speed that the other vehicle is travelling at and this is callibrated on a regular basis as are the tachometers/spedometers in the car.
If there is enough evidence for a person to be issued with a ticket then a ticket will be issued, the person then has the right to contest this at court, if s/he has any evidence then this can be produced. The judge will look at the police evidence, which is normally sealed after the incident to prevent any arguments of tampering or any evidence that the defendant may offer.
As was stated previously, the police do not find anyone guilty, they collect evidence and present it to the court who decide whether that person is guilty or innocent.
And as wired says, never argue with a police officer, it just f***s them off (i'd know)!!
Seat Ibiza SX mk IV
Progress 40% - all the kit, not enough time to install it!