Hey Chris, maybe this will help:
 
Weasels: White belly with a clear "line" separating the colored sides from
the white belly.
 
Ferrets: belly same as side color or even darker in the middle of the
abdomen.
 
Weasels: Tail either shorter (short-tailed weasels or stoats) than 1/3rd
body length, or longer (long-tailed weasels) than 1/3rd body length.  Tail
usually has a black tip (not in the least weasel which is short and of a
uniform color).  Tail skinny.
 
Ferrets: tail about 1/3rd body length.  Tail uniform in color.  Tail
tapered.
 
Weasels: Ears usually the same color as the rest of the head.
 
Ferrets: Ears usually have a white tip in dark colored animals.
 
Weasels: Body excluding tail equal to or shorter than about 7 inches.
 
Ferrets: Body excluding tail equal to or longer than about 7 inches.
____
 
What isn't a good way to tell the difference:
 
Size: Female ferrets and male long-tailed weasels are about the same size.
 
Mask: While the ferret mask is usually more pronounced, some long-tailed
weasels have strong mask patterns and some ferrets have poor ones.
 
Foot Marking: Some ferrets have darker feet that weasels, but depends on
the color pattern.
 
These are the best ways to tell over the phone.  Hope it helps.
 
Bob C and 16 MO' Ferrets of Freedom
[Posted in FML issue 2768]