Tool to find out if a MP3 has been re-encoded

Just wanted to know if there is a tool or way to find out if a MP3 file has been re-encoded (like CD->MP3 128kbps->MP3 320kbps).
I had some files i ripped from CD to 128kbps and my Ipod would not play them so i re-encoded them to 320kbps (converted the 128kbps MP3 file to a 320kbps MP3 file in winamp). I couldnt juts re-rip the CD because the original CD was from a rental store (ripping and making copies of rented stuff is 100% legal where i live). Now i just want to know which files i re-encoded and am searching for some kind of tool to do this.