I've read that the Spectral Analysis in Cool Edit can display if a wave file was converted from mp3 format. Audacity has a Spectral Analyzer, but I don't know what settings to use, and how to interpret them. Any help, please?

Just click Analyze -> Plot Spectrum... (first 28.3 sec. should be fine) and look at the graph. If you see a sharp drop like the one at 16000 Hz in the image below, you likely have a wave converted from mp3 (since the higher frequencies have been cut off in the encoding process and converting back to wave can't restore them of course). This example is a 128kbps mp3, with higher quality (=bitrate) mp3's this may not be so clear however.

Thanks for the info. I have a question - the reason I was unsure of the filters is because when you change the default "Hanning window" to "rectangular," the graph expands and goes beyond the previous reading on the same scale.

I have 2 cdrs from 2 different guys, supposedly both are duplicates of an original commercial cd, but one is louder than the other. When I Analyze them in Audacity, they both show the same 16 khz cutoff. Obviously at least one of them is fibbing, unless it's possible the retail cd could have this cutoff?

I'm sorry, I don't know the bakground of using the different "windows" for the spectrum, but any signals shown in the high frequency area (above the cutoff) are likely spurious/phantom signals and not part of the music. The "Hanning window" in my screenshot shows something there as well, although at a -80 dB level it's clearly not audible.

Anything is possible, but I wouldn't expect a retail CD to show a cutoff at 16 kHz like this.

I attached an image. I just thought since the volume was alittle low, perhaps the company might have done a poor job in mastering the cd. They're called "Label X." If not, then I guess I can't trust my fellow cd trader.