Check their teeth they should be a nice and white. Check the molars for food build up, or brown which may cause decay. Make this check up part of the routine at your veterinarians office.
Keep their nails trimmed, this will keep them from getting caught in something and doing damage to the toes.
Hmmmm...What do I have here? Many people don't know how to tell a boy from girl, and sometimes will tell you the wrong sex rather then admitting they don't know.
Gently rub you hand over you ferrets belly, do you feel a belly button? Congratulations, you have a boy, no belly button then you have a girl!
If you have a female the first signs of an incomplete spay will be her vulva swelling. This located below her tail and rectum, it is the one closest to her front.
With some females, this is the only symptom they will exhibit, others may suddenly seem a little aggressive towards the others in their group if you have more then one. They may go off on their own and start gathering nesting materials, shirts and other material, separated from their normal sleeping area.
The easiest way to find out if she's going into heat, is to check her weekly, just take a peek nothing more embarrassing then that. If you notice the vulva swelling, starting to look like a small life saver, bulging and getting pink, call the veterinarian. This will be life threatening if left unattended. A female ferret will not go out of heat by herself.
A male can also have an incomplete neuter. This is a little harder to detect, but the two most common signs are aggression and frequent urination.
If you only have one ferret it is hard to recognize, but if you have others you will see him attacking them, trying to mount, trying to drag them off. This sexual behavior play is different then normal rough play, it is much rougher and more persistent.
Raisins, a big ferret favorite, will generally come out looking like raisins. If diarrhea persists for more then a 24 hour period and several meals, it could be a sign of ECE, this needs immediate medical attention. During any episode of diarrhea a ferret still needs to eat and take water. If he's not drinking on his own, you'll need to force him.
A good thing to have on hand is pedialtye. In a pinch I use Gatorade, this will keep them hydrated, a ferret can get dehydrated in a very short time and this is dangerous.
To tell if your ferret is dehydrated pinch the skin and if it sticks together or does not bounce back he is dehydrated. Get him to a veterinarian IMMEDIATELY.