[jaygs]

I assume you have a tool that can show things in spectral mode. If so, just take a wav file and encode it in MP3 at 128 kbps. Then open them both in spectral mode. The mp3 file will have no "color" at the top of the spectrum -- higher frequencies. Old, low quality, but lossless files will have some color at the top, but it will be "faded" because it's just the hiss. It looks a little like old fashioned television "snow"