A tone's frequency is the number of times that the sound wave passes a given point in one second. The standard unit of frequency is hertz, abbreviated as Hz. One sound wave cycle per second equals one hertz. Counting wave cycles has traditionally been done with a sound sensor and a data logger, but you can now measure tone frequencies with your computer.
Open a sound analysis program on a computer with a sound card. Sound programs are available for free or inexpensive download (see Resources).
Adjust the sampling parameters of the analysis software to fit your preferences. A sound can be mathematically represented as sine waves in the Fourier transform; there are many ways to calculate a Fourier transform, but the quickest of these programs use a Fast Fourier Transform algorithm to analyze the components of your tone. Some programs include an oscilloscope to further analyze the sound.
Play the tone into the computer's microphone or open it as a .wav file in the analysis program. Launch the analysis.
Read the graph of the sound sample to find the frequency of your tone.