EncSpot is an application which reports useful facts about your mp3 collection.
The most famous feature is its ability to guess which encoder was used to encode each file. It will also give you a general idea of the audio quality of the file.
For files encoded with Lame (and that's what everyone's using, right?) EncSpot can read the 'Lame Tag' which gives detailed information on the switches used when the file was encoded.