Spek is a free audio spectrum analyzer that displays a visual representation of the frequencies contained in an audio file. It can be useful as a quick quality-control tool when checking music files, for example to spot frequency cutoffs that may indicate a lower-quality or compressed source.
A spectrum should be treated as one indicator rather than definitive proof of audio quality.
This tool is covered in The Loop Academy - Month 03 for assessing audio quality when preparing song files.