Ferrets do get plaque and tartar buildup on their teeth.  You can see
it as dark patches on the cheek teeth if you gently lift the ferret's
upper lip.  You can help control it by brushing their teeth with a pet
enzymatic toothpaste and a small cat toothbrush at least twice weekly,
especially after sticky or sugary treats.  The dry food most ferrets
eat also helps to keep the teeth clean; ferrets eating soft food on a
long-term basis will need their teeth cleaned more often.

However, most tartar and plaque starts out under the gumline, and it
takes a proper cleaning by a vet to get it off.  The job will be
easiest and most thorough if the ferret is under anesthesia [12.5]
during the cleaning; ferrets tolerate isoflurane very well, and the
risk from anesthesia is very slight.  A professional cleaning should
be done every one to three years, depending on how dirty the teeth
get.