The Doppler Effect for sound is the change in frequency or pitch that you hear from a moving source. It will be either higher or lower than the emitted frequency, depending on the direction the source is moving.
The Doppler Effect equations allow you to calculate the frequency of the sound as a source moves toward you, away from you, or even at an angle with respect to the line-of-sight. You need to know the initial frequency, the velocity of the source and the speed of sound, as well as the angle between the source and the line-of-sight.
Although you can hear the difference in frequencies, you really need to use a microphone and a frequency meter, such as an oscilloscope, to determine the initial and final frequencies.