That is where spectrum analysers come into play.
There are different kinds of analyser, but the most common after the humble VU meter is the frequency analyser.
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Here is a list of the most popular freeware analysers available:
Voxengo SPAN
Bluecat Audio Freqanalyst
Roger Nichols Inspector
For a long list of other free analysers, visit http://www.gersic.com/plugins/index.php?daCat=2

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