At our house when we have identical twins I clip the tip of the tail hair
blunt on one so I can tell at a glance who is who.  Works well with all
colors.  When we have 3 or more I clip fur from other spots as well,
beginning of tail by back (works on darker colors), top of head (flat top
works on all colors), side of back hip (dark colors unless you go down to
the skin or your white ferret has longer fur).
 
Hugs to all.  tle (aka Sprite)
[Posted in FML issue 1675]