There are several methods of manual speed detection, but they are all based on obtaining a known distance and comparing it to the time it took to travel that distance.    Officers who have been watching traffic for a long time tend to get a really good grasp of almost instantaneously knowing how fast vehicles are travelling.  A table showing an officer's guess along side an inexperienced observer's guesses and the radar results is shown below.  Note that Officer Keeler's guesses are all within 3 mph of the actual speed.  My guesses tend to have a higher deviation and were sometimes influenced by officer Keeler's guesses if he spoke first.

The other methods include using a stopwatch and timing a vehicle on a known distance, either from the ground or in an aircraft, and pacing a vehicle.  Pacing a vehicle involves either matching speeds with the vehicle and checking your speedometer or locating distinct markings and counting so that you can again divide distance by time to obtain a speed. 

Here, after the officer guesses your speed, he verifies it by using his radar gun.