Check when you clean your ferret's ears, perhaps once a month [7.5].
Reddish-brown ear wax is normal, but if you see any thick, black
discharge then you probably have mites.  It's also a good idea to have
your vet check the ears whenever you visit.  You can't catch ear mites
from your pet, but your cats, dogs, and other ferrets certainly can.

Dr. Williams, DVM says:

    Ferrets very commonly get ear mites, so you don't need to get
    upset.  Check with your vet and get two products: a ceruminolytic
    (such as "Oti-Clens"), which will dissolve the wax that the mites
    live in.  This is far preferable to trying to dig the wax out with
    Q-tips.  Then get a good ear miticide from your vet (I use
    Tresaderm).

    Put a little of the ear cleaner (which dissolves the wax) in the
    ear and massage.  Let it sit for about a minute.  Your ferret will
    probably shakes its head, sending wax all over you and the floor.
    Use a Q-tip and gently collect the rest of the wax from the ear
    canal.  You won't hit the ear drum, as the ferrets ear canal is
    roughly L-shaped - you will just be cleaning the vertical part of
    the canal.  After you have cleaned the wax, put the ear drops
    [miticide] in.  Make sure that the fluids that you are using are
    body temperature - put them in your shirt or pocket for a few
    minutes before using.  No one likes cold water in their ears!!!!

    Clean every day for a week to 10 days, stop for a week, and go
    again for another week to take care of mites.  If your problem is
    just dirty ears (some ferrets have a lot of wax) - just use the
    ear wax remover once a week.

    Ivermectin can be used in bad cases, either orally, injected, or
    directly in the ear.  Today I ran across an article (Bell, JA.
    Parasites of Domesticated Pet Ferrets, Comp. Clin. Educ. Pract.
    Vet. 16(5): 617-620), which gives a dosage for topical
    administration of ivermectin:

    Injectable ivermectin is mixed with propylene glycol at a ration
    of approximately 1:20 - then 0.2 to 0.3 ml (4-6 drops) into each
    ear canal daily.  Ferrets on ivermectin for heartworm prevention
    should not have problems with ear mites.

Dr. Susan Brown, DVM says:

    Do not depend on the oil [used for cleaning] to completely rid
    your pet of mites either although it will help to suffocate them.
    Mites are easily taken care of by using Ivermectin directly in the
    ears at 0.5mg/kg divided into two doses to be used in each ear and
    then repeated in two weeks.  You need to have a positive diagnosis
    of mites made by your vet and get the medication from him or her.

On at least two occasions, Oterna ear mite drops from Pitman-Moore Ltd
England (containing betamethasone BP, neomycin BP and monosulifiram)
have caused damage to the (outer) ears of ferrets, necessitating the
surgical removal of a portion of the ear.  It is recommended to avoid
using this medication for ear mites in ferrets, and to check other
medications for those ingredients.