Here is a pretty decent method on determining if your show may be MP3 sourced. Most MP3 encoders out there
cutoff the higher frequencies, so if we examine the audio frequency using a audio frequency spectrum analyzer we should see this.
The pictures below illustrate the various sourced shows.

Note that FM/TV and Mini Disc sourced shows suppress the higher frequencies.

Another note is that the newer MP3 encoders actually do a better job of softening the frequency cutoff. So this is not a sure way of
determining if your show may be MP3 sourced, but a start.

A link to various audio frequency spectrum analyzers you can download and use for yourself.

MP3 Sourced Show

Non MP3 Sourced Show

FM Sourced Show

Mini Disc Sourced Show