Q: is it rpm's? how can they tell?
A: The engine has a a tachometer that counts the engines R.P.M. (revolutions per minute). In any particular gear once the speed is established it will determine how fast the car is moving, at that certain R.P.M., as long as the transmission is'nt slipping. An example would be at 1800 R.P.M. the car is moving at 55 m.p.h. NASCAR monitors pit row speed with sensors embedded in the asphalt.