Thanks Adam, Ben, and others...
found the magic number file in
/usr/share/file/magic
(on OS X, but likely in the same place on any *nix, I'm guessing it's  
one of those files that is often used by people more sophisticated  
than myself who write C for a living)
There is a LOT of stuff in there!!
Wish I had looked in there before!
So I've written a minimal bit of Ruby like an lazy person. Copying  
D.Berger's Ptools style basically, by simply adding to the File class  
my mini methods.

Though I'm going to need some testing... the magic file (not always  
easy to read)
says this :
# MPEG 1.0 Layer 3
0       beshort&0xfffe  =0xfffa         \bMP3

I'm not 100% sure, but Adam said 0xFFFB or 0xFFFA, and the magic file  
lists only FFFA or does it mean FFFE and/or FFFA ?