Perhaps one of the easiest ways to approach the Board is with a text
in hand...and the one I would recommend is "Walker's Mammals of the
World"...any edition (I believe the newest is the 5th, although any
edition will work...but obviously the newer the better).  This 2 Vol set
is the mammologists "bible" and recognized worldwide as such.  Rat's,
mice etal will be found in Vol one under "Rodentia" as will ferrets under
"Carnivora".  That should end any further discussion on the matter...but
just in case...one of the easiest ways to separate ferrets (or any
carnivores) from rodents is by looking at the dentition.  This alone is
so definitive as to be irrefutable.  Of course then you need a skull or
two to present the board (just kidding...there are many mammal field
guides available in a library that have skull and dentition photos).
 
If they need reference from a wildlife biologist, you may have them
contact me...I'd be happy to help out...
 
putorius
[Posted in FML issue 3965]