Frequency comparator

The function of determining a signal's frequency content can be accomplished several different ways. If you just have a single frequency that you want to compare an input signal's spectrum against, you can use a simple bandpass filter to tell you how large the signal content is at that frequency in the input waveform.

If you want more information about multiple frequencies in the input waveform, you can use an analog spectrum analyzer circuit (mixer + LPF), or you can digitize the waveform, and use digital signal processing (DSP) and/or Fast Fourier Transform (FFT) processing to give you the signal energy at different frequencies.

As to the RADAR gun question, it is using the Doppler shift in frequency of the return signal to calculate the speed of the reflecting object.

http://en.wikipedia.org/wiki/Radar_gun