Ferrets are susceptible to a number of conditions, especially as they age. They tend to not show when they are in pain, however, there are a number of signs that you can watch out for which may be an indicator or symptom of something your ferret may be developing. When in doubt, it's always best to consult your veterinarian.
Bulging or swollen eyes: Glaucoma - must be treated with eye drops, this is an extremely painful condition.
Runny or watery eyes: Allergy, flu
Pus: Conjunctivitis- requires antibiotics
Brown crusting: Distemper - Quarantine immediately, 100% fatal in ferrets - humane action is to euthanize.
White spot or "disk" in eye/s: Cataract - if onset before 1 year old - treat with eye drops to prevent glaucoma
Blindness: Cataract, congenital. Retinal degeneration caused by taurine deficiency because not fed proper food for ferrets.

Sore on Nose: Infection
Runny nose, sneezing: viral infections, sneezing only - dust balls...
Short, broken whiskers; Poor nutrition, ill health
Dark pink or red; Insulinoma seizure - rub honey into gums, followed immediately by high protein food, see vet
Bright red;  Hypothermia (heat stroke) - medical EMERGENCY, needs veterinary treatment to prevent death

Black inside: Excessive dirt - Gently swab with Q-tip moistened in ferret ear cleaner
Black inside; itching Mites - see vet for appropriate treatment - DO NOT use over the counter pet store medications!
Yellow: Hepatitis - requires veterinary assessment for underlying cause
Bad smell Excessive dirt; yeast-type infection - see vet for appropriate treatment
Growth: Tumor; infected bite - see vet
Loss of hair on ears: Another ferret chewing on them or licking them
Deafness: Congenital, Wardenburg's Syndrome (panda pattern); infection from mite infestation

"Splayed" feet: Housed on wire - always cover ramps with cloth, and never house ferrets directly on wired floors
Hair loss; Age, adrenal disease - see vet for appropriate treatment if adrenal disease is suspected
Dry pads: Clay litter - substitute wood stove pellets or Yesterday's News
Dark pink pads: Insulinoma seizure - see vet for treatment
Bright red pads:  Hyperthermia (heat stroke) - medical EMERGENCY - seek veterinary intervention to prevent death
Rough, thickened pads: Distemper - Quarantine immediately, seek veterinary interventon
Roughened, scratched pads; Lost outside for significant period; housed outdoors/dirt/cement
Long quick; Not regularly clipped - trim nails once a week
Nail torn out: Caught on bedding; on cage wire - trim nails once a week to prevent
Curled toes: Age, weak hindquarters

Limping: Stepped on - x-ray to rule out fracture
Staggering: Insulinoma; stroke; ear infection
Stiffness: Arthritis; age; over-caged
Weakness in hindquarters: Age; arthritis; low blood sugar; general illness

Kinked: Previously broken; birth defect
Lump at end: Chondromas/chordomas - need to be surgically removed to prevent tumor spreading.  It is also a very painful condition for the ferret because the tumor disintegrates bone.
Black spots: Blackheads - use ferret spray-on shampoo and rub in once a day until blackheads gone
Hair loss: Blackheads; age; seasonal shedding, adrenal disease

Fur & Skin
Dry: Sleeping in litter; poor nutrition; adrenal disease, age; general illness
Black spots: Flea dirt
Fur falling out: Seasonal coat change; poor nutrition; stress; adrenal disease; flea allergy
Guard hair sparse: Regrowth of coat
Blue/black patches on skin:  Regrowth of coat
Thinning: Age; unnatural light cycle; adrenal disease; in heat or rut
Pattern Loss: At base of tail Adrenal disease
Along side back: Adrenal disease
Up back: Adrenal disease
Over shoulders: Adrenal disease
"Graying": Natural silvering pattern; age
Flaking skin: Flea dermatitis; poor nutrition; allergies; over-shampooing
Yellowed: Hepatitis
Reddened, rough skin: Allergies; fleas; sunburn; bites from others; adrenal disease
"Pinched" skin stays pinched: Dehydration - requires Sub-Q hydration
Wart-like growths: Skin tumors
Dark brown or black "moles": Ticks
Spot that looks like a large blood blister, bleeds off and on:  Hemangiosarcoma: requires excision, prognosis good if removed.
"Puffy," inflamed areas: Abscess
Sore slow to heal: Diabetes; adrenal disease; cancer

Coughing: Flu, heart disease; lymphosarcoma
Wheezing: Allergy; heart disease; pneumonia
Rapid breathing: Heat stroke; pain
Difficult breathing Heart disease; pneumonia; lymphosarcoma; heartworm

Stomach distension: Intestinal blockage; heart disease; poison; internal tumor
"Lopsided" abdomen: Enlarged spleen
Hardened area along abdomen: Enlarged spleen
Anorexia: Stomach or mouth ulcers; ECE

Male "dribbling" or crying: Urinary tract blockage; bladder stones; kidney infection; adrenal disease - Blockage is a medical EMERGENCY, without treatment ferret will die a horribly painful death within 10 hours
"Sand" in urine: Bladder stones
Heavy urination &/or excessive thirst: Diabetes; kidney infection, renal failure
Inability to urinate: Kidney failure; blockage - Medical EMERGENCY
Inability to defecate: Internal blockage - requires surgical intervention to remove obstruction
Diarrhea: Dairy products; food allergies; stress; coccidia; camphobacter, poor diet
Green diarrhea: Upset stomach; stress; ECE
Dark, tarry stool: Too many raisins (DO NOT FEED RAISINS - can lead to renal failure); bleeding from ulcer
Blood in stool: Internal hemorrhage, cancer
Fabric or odd objects in stool: Eating fabric or toys - Give hairball remedy until cleared.
Frequent, small stool: Poor rectal muscles
Protrusion from anus: Rectal prolapse
Draining area near anus: Anal gland impaction

Reproductive Organs
Bleeding penis: Caught on something
Redness anywhere on penis:  Urinary tract infection, stones, or enlarged prostate due to adrenal disease requires medical attention asap!
Growth on penis: Tumor
"Dragging" penis across things: Territorial marking; adrenal disease
Swollen vulva: In heat; leftover ovarian tissue; adrenal disease
Pus or discharge from vulva: Infection; adrenal disease
Difficulty urinating in a male: enlarged prostate due to adrenal disease, could lead to blockage, which is a medical emergency. You have 10 hours to get him to a vet or he will die a painful death.

Overall Body
Temperature over 39.4C (103F): Fever
Temperature under 35C (95F): Hypothermia; serious illness
Severe anemia: Lengthy estrus cycle; internal tumor; severely enlarged spleen
Limp upon awakening: Baby sleep; low blood sugar; insulinoma
Comatose: Insulinoma; stroke
Uncontrollable shivers/tremors: Shock; poison
Shivers brief:  Waking up; excitement
Convulsions: Insulinoma seizure; epilepsy; poison; shock
Wasting: Cancer; age; internal blockage; internal parasite
Enlarged lymph nodes: Infection; flu; lymphosarcoma
Blood sugar level under 4.5: Anorexia; insulinoma

Fainting: Insulinoma or heart block
Lethargy: Anemia; low glucose; age; heart disease; illness
"Listing " to one side: Ear infection; mites; stroke
Walking in circles: Stroke or central nervous system/brain tumor
Excessive grooming: Stress; adrenal disease
Biting when startled: Blind; deaf
Biting other ferrets: Blind; dominance issues; pain

Hopping & leaping PLAYING!

Pale gums: Anemia, low blood pressure
Red, sore or bleeding gums: Tarter buildup; gingivitis - requires dental scaling
Bluish gray gums: Lack of Oxygen
Blackened teeth: Dead tooth - may require extraction
Dark or stained teeth: Age, tetracycline as kit
Rash on chin and lips: Distemper - Quarantine immediately, 100% fatal in ferrets - euthanize to prevent horrible suffering
Drooling: Insulinoma seizure; poison - rub honeyKaro into gums, immediately followed by high protein food i.e., A/D
Clenched teeth: Insulinoma seizure - requires veterinary intervention and treatment
Scratching at mouth: Broken tooth, nausea
Vomiting: Foreign body, ulcers, ECE; hairballs; poison - EMERGENCY take to vet!
Vomiting blood: Internal hemorrhage - EMERGENCY take to vet immediately!
Panting: Heat stroke; severe pain; insulinoma seizure - Heat stroke is a medical emergency get to vet

For more detailed information on how to deal with a ferret emergency go to:

Ferret Biochemistry and Hematology Values
Blood Glucose Conversions