[MP3 ENCODER] Detecting bad MP3's?

[Intimidated]

Hi all.   I am currently working on a new file sharing app, on the behalf of the folks at www.unitethecows.com. I have a question:   Is there a way to detect an MP3 file which has been re-encoded from a lower bitrate? For example, if somebody gets a 24k mp3 stream, and re-encodes it at 128k, can that be detected?   I've observed that if you compress a 128k stream, it'll compress more if it used to be a lower bitrate.. but I can't simply use this to code by.   Thanks for your help!   Intimidated.